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3

Networks

section A

NETWORK BASICSSECTION

COMMUNlCATlON SYSTEMS
You use many networks for communication, research, and entertainment. Some networks are large and some are small. The largest networks offer little control to consumers. Smaller networks that you set up can be com- pletely under your control, but they are also your responsibility. Networks can be classified in many ways; as a network user, you'll want to keep in mind the idea of control and how it effects your privacy and security.
◗What is a network?A network links things together. A communica- tion network (or communication system) links together devices so that data and information can be shared among them.
in 1948, Claude Shannon, an engineer at Bell Labs, published an article describing a communication system model applicable to networks of all types. His diagram illustrates the essence of a network, so it is a good place to begin this module. Shannon's model (Figure 3-1) is easy to understand.

◗How are computer networks classified? Networks can be classi- fied according to their size and geographic scope, as shown in Figure 3-2.


◗Why is geographic scope important? Localized networks nor- mally include a small number of computers, which can be connected using basic equipment. As the area of network coverage expands, the number of devices grows, specialized devices are sometimes required to boost sig- nals, and the diversity of devices requires sophisticated management tools and strategies.
◗What about the lnternet of Things? The lnternet of Things (loT) is an evolving concept that may be difficult to classify as a PAN, LAN, or WAN. The loT has the potential to become a global collection of smart devices transmitting to other devices over the lnternet. Today, smart devices are more often grouped into small local pods that report to a centralized device, which in turn exchanges data with local networks and the lnternet.
COMMUNlCATlON CHANNELS
Do you suppose it is easier for a snooper to surreptitiously access your computer when your device is connected to a Wi-Fi hotspot or cabled to a LAN? Some connections are more secure and more dependable than oth- ers, so it pays to understand the ins and outs of communication channels.
◗What is a communication Channel? A communication chan- nel is the medium used to transport information from one network device to another. Data transmitted over a communication Channel usually takes the form of an electromagnetic signal—waves of light, electricity, or sound. These waves can travel through the air or through cables, so channels are divided into two general classifications: wired and Wireless. Wired chan- nels transport data through Wires and cables. Wireless channels transport data from one device to another without the use of cables or Wires.
◗What are the options for wired channels? Wired channels include twisted pair Wires used for telephone land lines, coaxial cables used for cable television networks, Category 6 cables used for LANs, and fiber- optic cables used for high-capacity trunk lines that provide main routes for telephone, cable, and lnternet communications (Figure 3-3).

◗What are the advantages of wired channels? Before wireless technologies became available, local area networks were exclusively wired. Today, wired connections are used less frequently for home, school, and business networks. They remain the network technology of choice, however, for segments of the Internet and local area networks that require fast and secure connectivity. When you set up a wired connection, you don’t have to worry about hackers intercepting your data from the sidewalk outside your house, or your neighbor stumbling across personal files when your wireless signal reaches past your property line. Yes, there are ways to tap into a wired network, but they require physical access to the cable or fairly sophisticated snooping equipment. The advantages of wired channels are summarized in Figure 3-4.
◗What are the disadvantages of wired connections? The cables that offer speed and security for a wired connection are also the main weakness of this type of connection. The disadvantages of wired channels include cost, lack of mobility, and installation hassles. Figure 3-5 provides more details.
Cables can be shielded against interference and encased in protective casings for installations that are outdoors and underground.
Wired connections are dependable. Their carrying capacity and speed are not affected by airborne interference from rain, snow, or electrical devices.
Wired connections are more secure than their wireless counterparts because a device can join a wired network only if it is physically connected by a cable.





◗What are the options for wireless channels? The most wide- spread wireless channels for communication networks are radio signals and microwaves.
◗How do radio signals transport data? Most wireless channels transport data as RF signals, commonly called radio waves. RF chan- nels are typically used for Bluetooth connections, Wi-Fi networks, and wide area wireless installations such as WiMAX. it is also the technology used to carry voice and data between a smartphone and a cell tower. RF signals are sent and received by a transceiver (a combination of a trans- mitter and a receiver) that is equipped with an antenna (Figure 3-6).

◗How do microwaves transport data? Microwaves (the waves themselves, not your oven!) provide another option for transporting data wirelessly. Like radio waves, microwaves are electromagnetic signals, but they behave differently. Microwaves can be aimed in a single direction and have more carrying capacity than radio waves. However, microwaves can- not penetrate metal objects and work best for line—of-sight transmission when a clear path exists between the transmitter and receiver.
Microwave installations usually provide data transport for large corporate networks. They are also used to transmit signals between towers in cellular and wide area wireless installations.
◗What are the advantages and disadvantages of wireless? The main advantage of wireless connections is mobility. Wireless devices are not tethered to network cables, so battery-operated laptops, tablets, and smartphones can be easily moved from room to room, or even out- doors. With wireless networks, there are no unsightly cables, and power spikes are much less likely to run through cables to damage equipment. The main disadvantages of wireless channels are speed, range, security, and licensing.
◗Why is Wireless slower than Wired? Wireless signals are sus- ceptible to interference from devices such as microwave ovens, cordless telephones, and baby monitors. When interference affects a wireless sig- nal, data must be retransmitted, and that takes extra time.
◗What limits the range of a wireless connection? The range of a wireless signal can be limited by the type of signal, the transmitter strength, and the physical environment. Just as radio stations fade as you move away from their broadcasting towers, data signals fade as the distance between network devices increases. Signal range can also be limited by thick walls, floors, or ceilings.
Terminology RF stands for radio frequency.


NETWORKS
As signal strength decreases, so can speed. Aweak signal usually means slow data transfers. You can get a rough idea of signal strength for your desktop, laptop, tablet, or smartphone by checking the network signal strength meter (Figure 3-7).

◗What's the problem with wireless security? Wireless signals float through the air and penetrate walls. The signals that carry your wire- less data can be accessed from outside your premises. Someone outside of your house, for example, could surreptitiously join your network, access files, and piggyback on your lnternet connection. To make wireless data useless to intruders, it should be encrypted. Later in this module, you'll learn how to use encryption to secure data sent over wireless connections.
◗How does licensing affect wireless connections? Government agencies, such as the Federal Communications Commission (FCC), regu- late signals that are sent through the air. To broadcast at most frequencies, including those used by radio and television stations, a license is required.
Wireless connections use unlicensed frequencies that are available for pub- lic use. These frequencies include 2.4 GHz and 5 GHz. Of the two, 5 GHz is subject to less interference from other devices, but it has a more limited range.
◗What's bandwidth? Network Channels must move data and move it quickly. Bandwidth is the transmission capacity of a communication Channel. Just as a four-lane freeway can carry more traffic than a two-lane street, a high-bandwidth communication Channel can carry more data than a low-bandwidth Channel. For example, the coaxial cable that brings you more than 100 Channels of cable TV has a higher bandwidth than your home telephone line.
The bandwidth of a Channel that carries digital data is usually measured in bits per second (bps). For example, your wireless LAN might be rated for an average speed of 27 Mbps. The bandwidth of a Channel carrying analog data is typically measured in hertz (Hz). For instance, the copper wires that carry voice-grade telephone signals are often described as having a bandwidth of 3,000 Hz.
As of 2015, the FCC defines networks that are capable of moving at least 25 megabits of data per second (25 Mbps) as broadband. Channels slower than 25 Mbps are classified as narrowband. Broadband capacity is essential for networks that support many users and those that carry lots of audio and video data, such as music and movie downloads.
85



NETWORK TOPOLOGY
A spider weaves a web by making silky connections between leaves, branches, and other surfaces. Most spiderwebs have a structure, and the same can be said for communication networks. The topologies of the net- works you use have an effect on their dependability, security, and scope.
◗What is network topology? in the context of communication net- works, topology refers to the structure and layout of network components, such as computers, connecting cables, and wireless signal paths. When you envision how devices are connected by communication channels, you are creating a picture of the network's topology.
Module 2 explained how peripheral devices can connect to a host device using expansion ports, USB cables, or Bluetooth. These connections are an example of point-to-point topology. When connecting several devices, two network topologies are popular. Star topology connects multiple devices to a central device. Mesh topology connects multiple devices to each other, either as a full mesh or as a partial mesh. The less popular bus topology connects devices in a linear sequence. Figure 3-8 illustrates these network topologies.

◗Can a network use more than one topology? Data can flow over multiple networks that have different topologies. As shown in Figure 3-9, data from a fitness wristband connects to a laptop via a point-to-point con- nection (A). The laptop is part of a home network configured as a star (8). The home network uses Comcast, so it is part of a larger star network (C). Finally, the date is passed to the lnternet, which has a mesh topology (D).


◗Which topology is best? Every topology has strengths and weaknesses, so there is no best network topology. Figure 3-10 compares strengths and weaknesses of the two most popular network topologies based on dependability, security, capacity, expandability, control, and monitoring.
Dependability
If the central point fails, data cannot flow anywhere on the network. If one of the other devices fails, however, the rest of the network remains operational.
There is no central point of failure; redundant paths between devices can be used to bypass failed devices.
Security
Data that travels on a star pathway makes only one stop between the sender and destination. The threat area for any transmission encompasses only three devices and two channels.

Within a mesh, data travels through several devices and over multiple channels. Each leg presents a potential security risk. The chance of a security breach rises as the number of devices and channels increases.
Capacity
Star topologies are limited by the amount of data that can be handled by the central device.

Mesh topologies offer higher capacities because data can be transmitted from different devices simultaneously.
Expandability
Expandability is limited by the number of devices that can be attached to the central device within its immediate area of wireless coverage or maximum cable length.
The network can be expanded infinitely. As new devices are added, the network continues to repeat the signal as necessary until it reaches the farthest devices.
Control
Setup and updates are primarily done on the central device, which also can be used to shut down the entire network.
Setup is more complex, as each device must be configured to send, receive, and forward network data. There is no central point at which the network can be shut down.
Monitoring
All data passes through a central point, which is easy to monitor for legitimate or illicit purposes.
Data does not pass through a central point, making data more challenging to monitor.

Communication networks connect all kinds of devices: from smartphones to satellite dishes, from computers to cell towers, and even between tiny sensors and RFlD tags. Any device in a network is called a node. You are familiar with network nodes such as laptops, smartphones, tablets, desk- tops, and peripheral devices. There are many other nodes that you don't interact with directly, yet they ultimately control how smoothly your Netflix movie streams and whether your email arrives at its destination.
◗What should ! know about network nodes? Devices on a network are classified as DTEs or DCEs. DTE stands for data terminal equipment. A DTE can be any device that stores or generates data. When connected to a network, your laptop is a DTE, as are your smartphone, tablet, and fitness tracker. The servers that house Web sites, handle email, offer cloud storage, and stream videos are also DTEs. Your own DTEs are under your control, and many servers are open to public access.
DCE stands for data communication equipment. These devices control the speed of data over networks, convert signals as they jump from cables to wireless, check for corrupted data, and route data from its origin to its desti- nation. The most well-known DCEs are routers and modems.
◗How does a router work? You probably have a DCE in your home network. A router is a device that controls the flow of data within a net- work and also acts as a gateway to pass data from one network to another. Routers are used to direct traffic over major lnternet trunk lines. And they are commonly used to route data from home networks to the lnternet (Figure 3-11).

◗How does a modern work? A modem contains circuitry that con- verts the date-carrying signals from a digital device to signals that can travel over various communication Channels. The kind of modern you use depends on whether you are connecting to a dial-up, wireless, cable, sate!- lite, or DSL lnternet service. A modem is usually supplied by your lnternet service provider (Figure 3-12).



◗What about other Dces? DCEs perform all sorts of tasks. Suppose you want to extend the range of your home network out onto your balcony. What if your Internet provider wants to streamline network traffic in a neighborhood where lots of subscribers stream HD movies? DCEs such as repeaters, switches, and hubs can get the job done (Figure 3-13).

QUlCKCHECK
To extend the reach of your wireless network out onto your balcony, which of the following devices would be best?

1. A hub
2. A router
3. A repeater
4. A gateway
   

◗How can a network detect if a signal has been corrupted? Error correction is one of the responsibilities of communication protocols. Remember from Module 1 that text, numbers, sound, images, and video all are represented by bits. Suppose that some of those bits are garbled in transmission. Did that email say to meet at 29:OO? Did your music track stop halfway through? Without error checking, the data you receive may not be reliable or complete.
Digital networks—those that transmit digital signals—can be easily mon- itored to determine if interference has corrupted any signals. At its most primitive level, digital equipment is sensitive to only two frequencies—one that represents 1s and one that represents Os.
Suppose that 8 0 is sent as —5 volts and 8 1 is sent as +5 volts. What if, dur- ing transmission, some interference changes the voltage of 8 1 bit from +5 volts to +3 volts?
To correct the corrupted bit, the receiving device realizes that +3 volts is not one of the two valid voltages. lt guesses that 3 1 bit (+5 volts) was actually transmitted and cleans the signal by reestablishing its voltage to +5 (Figure 3-15).




BACKGROUND
The Internet has its roots in a U.S. Department of Defense project that transitioned first into a nonprofit civilian operation, and then to a burgeoning commercial enterprise. We use the Internet without a second thought to communicate, create, and consume content. Yet the Internet also offers profound ethical, security, privacy, and legal challenges. To understand these challenges and the controversies surrounding them, all the stakeholders need a good understanding of basic Internet technology.
◗how did the Internet get started? The history of the Internet begins in 1957 when the Soviet Union launched Sputnik, the first man-made satellite. In response to this display of Soviet expertise, the U.S. government resolved to improve its scientific and technical infrastructure. One of the resulting initiatives was the Advanced Research Projects Agency (ARPA). ARPA swung into action with a project designed to help scientists communicate and share valuable computer resources. The ARPANET, created in 1969, connected computers at UCLA, the Stanford Research Institute, the University of Utah, and the University of California at Santa Barbara (Figure 3-16).
In 1985, the National Science Foundation (NSF) used ARPANET technology to create a larger network, linking not just a few mainframe computers but entire LANs at each site. Connecting two or more networks creates an internetwork, or internet. The NSF network was an internet (with a lowercase i). As this network grew throughout the world, it became known as the Internet (with an uppercase I).
◗how did the Internet become so popular? Early Internet pioneers used primitive command-line user interfaces to send email, transfer files, and run scientific calculations on Internet supercomputers. Finding information was not easy, and access was limited to a fairly small group of educators and scientists. In the early 1990s, software developers created new user-friendly Internet access tools, and Internet accounts became available to anyone willing to pay a monthly subscription fee.
Courtesy of the Computer History Museum

QUlCKCHECK
The ARPANET was __________.

1. a network used by the Soviet Union’s space program
2. essentially a pilot program for what later became the Internet
3. created by the National Science Foundation

BACKGROUND
The Internet has its roots in a U.S. Department of Defense project that transitioned first into a nonprofit civilian operation, and then to a burgeoning commercial enterprise. We use the Internet without a second thought to communicate, create, and consume content. Yet the Internet also offers profound ethical, security, privacy, and legal challenges. To understand these challenges and the controversies surrounding them, all the stakeholders need a good understanding of basic Internet technology.
◗How did the Internet get started? The history of the Internet begins in 1957 when the Soviet Union launched Sputnik, the first man-made satellite. In response to this display of Soviet expertise, the U.S. government resolved to improve its scientific and technical infrastructure. One of the resulting initiatives was the Advanced Research Projects Agency (ARPA). ARPA swung into action with a project designed to help scientists communicate and share valuable computer resources. The ARPANET, created in 1969, connected computers at UCLA, the Stanford Research Institute, the University of Utah, and the University of California at Santa Barbara (Figure 3-16).
In 1985, the National Science Foundation (NSF) used ARPANET technology to create a larger network, linking not just a few mainframe computers but entire LANs at each site. Connecting two or more networks creates an internetwork, or internet. The NSF network was an internet (with a lowercase i). As this network grew throughout the world, it became known as the Internet (with an uppercase I).
◗How did the Internet become so popular? Early Internet pioneers used primitive command-line user interfaces to send email, transfer files, and run scientific calculations on Internet supercomputers. Finding information was not easy, and access was limited to a fairly small group of educators and scientists. In the early 1990s, software developers created new user-friendly Internet access tools, and Internet accounts became available to anyone willing to pay a monthly subscription fee.
Courtesy of the Computer History Museum

Source: 2015 PEER1

◗How big is the lnternet today? With an estimated 500 million nodes and more than 3 billion users, the lnternet is huge. Although exact figures cannot be determined, it is estimated that the lnternet handles more than two exabytes of data every day. An exabyte is 1.074 billion gigabytes, and that's a nearly unimaginable amount of data. Visualizing such a vast network is difficult, but images like those in Figure 3-17 help a bit.

◗Who operates the lnternet? Although the lnternet is such a vast entity, in theory no single person, organization, company, or government runs it. At one time, the lnternet was composed of many regional networks, established in countries throughout the world. Gradually, these networks have fallen under the control of large telecommunications companies, such as Comcast, AT&T, and NTT Communications.
The glue that holds the lnternet together and makes it possible for data to travel across borders is a set of standard protocols that were developed for the original ARPANET. in this respect, lnternet governance is Simply a set of shared protocols, procedures, and technologies that evolve through common agreement among network providers.
◗Who supervises lnternet standards? Although each country can develop laws, policies, and regulations for the networks within theirjurisdic- tion, there is one crucial administrative task necessary to keep the lnternet from sinking into chaos. Every lnternet node—each server, each computer, and each piece of data communication equipment—must have a unique address in order to send and receive data.
The organization that supervises lnternet addressing is lCANN, the lnternet Corporation for Assigned Names and Numbers. lCANN is a not-for-profit private sector organization operated by an international board of directors. lts advisory committee has representatives from more than 100 nations, and public comments pertaining to lCANN policies are accepted at its Web site.
Control of lnternet address assignments is a powerful tool. lt provides the means to block users from accessing specific network nodes. Local govern- ments exercise this power within theirjurisdictions to shut down servers that violate copyright and distribute inappropriate content. The power to globally shut down access to a server, however, only rests with lCANN. You will discover more about the key role of lnternet addresses later in this section.


INTERNET
iNFrAsTruCTure The Internet is not one huge communication network, but rather a network of networks. The way these networks fit together is referred to as the Internet infrastructure. Internet networks are organized into tiers. As a consumer, you are subject to the fees, policies, and technologies of the tiers that provide your Internet service. ◗What are the components of the Internet? The Internet is structured as a hierarchy of networks. Tier 1 networks are at the top of the hierarchy, followed by Tier 2 and Tier 3 networks. Tier 1 networks, such as AT&T, CenturyLink, Verizon, and NTT Communications, form the Internet backbone, a system of high-capacity routers and fiber-optic communication links providing the main routes for data speeding across the Internet. Routers on the Internet backbone store routing tables that calculate and track the most efficient routes for data to travel from point A to point B. The Internet backbone is configured as a mesh network that offers redundant routes for data transport. The backbone’s mesh infrastructure is probably the basis for the myth that the Internet originated as a Department of Defense project to create a network that could survive nuclear attacks. Networks that form the Internet are maintained by Internet service providers (ISPs) that supply routers and other data communication equipment, as well as physical and wireless channels to carry data. ISPs exchange data at Internet exchange points (IXPs). Consumers generally connect to Tier 2 or Tier 3 networks. Use the infographic in Figure 3-18 to become familiar with the Internet infrastructure and its terminology; pay attention to the arrangement of network tiers and points where data moves between them.


◗Who pays for the Internet? The Internet is not free. ISPs make a substantial investment in equipment and the cable or wireless infrastructures to connect consumers. The largest providers each have close to 200,000 miles of cables installed across continents and laid over the ocean floor. Tier 1 ISPs also own and maintain millions of dollars of data communication equipment. In addition to infrastructure expenses, ISPs are subject to data transport fees, especially when shipping data up through higher tiers. To offset expenses, ISPs charge consumers for access. Figure 3-19 explains how it works.

PACKETS
Most people envision their files, email, and other data flying through the Internet as a continuous stream of bits. This is not the case. Files are chopped up into small pieces called packets. The technology that breaks a file into packets and routes them to any location on Earth in the blink of an eye is absolutely amazing.
◗What’s a packet? A packet is a parcel of data that is sent across a computer network. Each packet contains the address of its sender, the destination address, a sequence number, and some data. When packets reach their destination, they are reassembled into the original message according to the sequence numbers (Figure 3-20).



◗Why not just send an entire message? Some communication networks, such as telephone systems, use a technology called circuit switching, which essentially establishes a dedicated, private link between one telephone and another for the duration of a call. Unfortunately, circuit switching is rather inefficient. For example, when someone is on hold, no communication is taking place—yet the circuit is reserved and cannot be used for other communications. A more efficient alternative to circuit switching is packet switching tech- nology, which divides a message into several packets that can be routed independently to their destination. Packets from many different messages can share a single communication channel, or circuit.
Packets are shipped over the circuit on a first-come, first-served basis. lf some packets from a message are not available, the system does not need to wait for them. lnstead, the system moves on to send packets from other messages. The end result is a steady stream of data (Figure 3-21 ).


◗How are packets created? One of the core lnternet protocols, TCP (Transmission Control Protocol) is responsible for dividing files into chunks, adding headers containing information for reassembling packets in their original order, and verifying that the data was not corrupted while in transit (a process called error checking). When data is sent over the lnternet using TCP, it will reliably reach its destination. TCP is built into applications that transmit data from one digital device to another.
◗How are packets transported? TCP is also responsible for estab- lishing a connection, transferring packets, and closing the connection when the transmission is complete. Most of the data that flows over the lnternet is under the control of TCP.
An alternative transport protocol, UDP (User Datagram Protocol) is faster than TCP but does not perform error checking and cannot reorder packets if they are received out of order. UDP, therefore, is suitable for applications in which a bit of lost data is not critical, such as streaming video and music, and lnternet-based multiplayer games and voice calls. UDP and TCP both use communications ports to shuttle data into and out of a network device.
◗What is a communications port? Here's the problem: On a packet- switching network, packets for Web pages, email, streaming videos, and other downloads may arrive at your digital device in the same stream and over the same Channel. The packets may not arrive in neat little bundles; some video packets could arrive interspersed with Web page packets. Which packets should go to the browser and which to the Netflix player?
A communication port (usually referred to simply as a port) is a virtual end point for data entering and leaving a digital device. These ports are virtual in the sense that they are not physical ports, as are USB ports, for example. A communication port is not a physical circuit, but rather an abstract concept of a doorway, an opening, or a portal through which data flows.
Ports work in conjunction with lnternet addresses, as you'll learn in later modules. Computers can have up to 65,535 ports. Typically about 10—20 ports are in use and open for various types of data. Data originating from the Web, for instance, uses port 80, whereas streaming video uses port 554. When ports are open, data is allowed to flow freely. Closing ports can block data, and that strategy is used by firewalls to prevent unauthorized intrusions.
Ports create simulated end-to-end connections on packet—switching net- works. So although the devices on two ends of a communication Channel are not connected by a single dedicated circuit, ports create a conceptual circuit for each type of data, as shown in Figure 3-22.


Port 80 Web browser
Port 554 Neffl'x player
Data pours into a digital device from Data with similar port numbers is multiple sources. channeled to associated apps.
NETWORKS
lNTERNET ADDRESSES
You might have heard that lnternet addresses are running out. Does that mean some people will be unable to get online? Will people have to share addresses? Networks use several kinds of addresses to determine where packets originate and where they are headed. To find out how lnternet addresses might affect your online access, read on. I Exactly What is an lnternet address? Although most people are accustomed to entering something such as www.wikipedia.org to access lnternet locations, those "www" addresses are not the underlying address used to transport data to its destination. lnternet addresses are controlled by lP (lnternet Protocol), which—along with TCP—is part of the lnternet protocol suite. lP defines two sets of addresses: v4 and v6.
◗What is the difference between v4 and v6?IPv4**, which stands for lnternet Protocol version 4, is the lnternet address standard that has been used since the early 19803. v4 uses 32-bit addresses to uniquely identify devices connected to the lnternet. ln binary, v4 addresses are written as:
11001111010010110111010100011010
For convenience, 32-bit binary addresses are usually written in decimal as four sets of three digits separated by periods:
207.75.117.26
Using 32 bits, v4 offers about 4 billion unique addresses. in 2011, that supply of addresses was completely allocated. Yes, lP addresses can be recycled when the original assignees no longer want them, but the demand far outstrips the number of addresses coming up for recycling. Another set of addresses was needed.
IPv6 (lnternet Protocol version 6) uses 128 bits for each address, producing billions and billions of unique lnternet addresses. An v6 address is usually written as eight groups of four hexadecimal digits, like this:
2001:4838:800:1192:198:110:192:54
Don't worry, there are enough v6 addresses for the foreseeable future, even with the anticipated influx of 2 billion new lnternet users and the lnternet of Things poised to add an estimated 50—100 billion devices to the lnternet by 2020. I Does every lnternet user need an lP address? it is more accurate to say that every device on the lnternet needs an lP address. Many, but not all, devices on the lnternet have permanently assigned lP addresses, called static lP addresses. As a general rule, routers and computers on the lnternet that act as servers use static lP addresses.
lSPs, Web sites, Web hosting services, and email servers that always need to be found at the same address also require static lP addresses. There is an emerging trend for individuals to request static lP addresses for their home networks, and a static address might be useful for remotely commu- nicating with sensors and other devices in the lnternet of Things.


◗What happens When a device doesn't have a static lP address? lP addresses can be temporarily allocated so that a device uses an address only while it is actively online. When the device is turned off or its lnternet connection is disabled, the address can be recycled to another device. The next time the device is turned on, it will be assigned a different lP address. lnternet addresses that are temporarily assigned are called When COHHECTEd to & network, dynamic lP addresses, your computer can receive an dd f ln practice, dynamic lP addresses do not change very often. Today, a ress rom most consumers have always-on lnternet connections that remain active 3- DHCP as long as the router that connects to the lnternet remains powered on. b. packets Turning a computer off or on does not affect the lP address stored in the router. Customers who access the lnternet using Comcast XFlNlTY and AT&T U-verse, for example, could have the same lP address for weeks or d. the CPU months, c. v4
◗How do devices get lP addresses? lP addresses can be assigned by a network administrator, but more commonly they are automatically assigned by DHCP (Dynamic Host Configuration Protocol). Most devices are preconfigured to receive an lP address by sending a query to the net- work device acting as the DHCP server. That device could be a router for a local area network or a DHCP server from your lSP. lP addresses get a bit tricky because a device can have a public lP address and a private lP address.
◗What is a private lP address? Let's suppose that your laptop is con- TRY "! nected to your school network. The DHCP server for the school assigns a Find your lP address. Is it in the dynamic lP address to your laptop when you log in. That address, which range of private addresses Iisted probably begins with 10, 172, 192, FD, or FcOO, is classified as a private lP in Figure 3-23? address because it works only within the school network.
A private lP address can be allocated by any network without supervision from lCANN. However, the address cannot be used to send data over the lnternet; it is not routable. Figure 3-23 demonstrates how to find your private lP address.

◗If my private lP address can't be routed over the lnternet, how does my data get anywhere? Here is where your local router plays a crucial role. Any network you use to access the lnternet—a campus network, your home network, or a Wi-Fi hotspot—has a router that con- nects to the lnternet. The router has a public lP address that is routable over the lnternet. Figure 3-24 explains how public and private lP addresses work.

TRY lT! What is your public lP address? You can find it by googling What's my lP. Compare your public lP address to your private address.

◗Does using a private lP address make me anonymous? No, the router's network address translation table keeps track of your activities, so your footprint across the lnternet can be traced back to you. However, a private lP address can protect you from some threats. For example, a hacker who randomly enters lP addresses looking for unauthorized access will never find your private lP address because the only address that is vis- ible to the public is the address of your local router. This security technique is usually referred to as NAT (network address translation).
That being said, private lP addresses cannot protect your devices from many other attacks. Clicking corrupted links or downloading infected files sends packets and also makes a path in the network address translation table that can allow malware into your device.
e'LTN
router's public lP address. -\er for 35.24.35_12
music store
Thriller to 35.2L-35'11 6 The music store sends the stream to your router.
QUlCKCHECK A local network's router uses an address table to keep track of private lP addresses.

1. lP
2. routing
3. translation
4. dynamics

◗What is a top-level domain? A domain name ends with an extension that indicates its top-level domain. For example, in the domain name msu.edu, .edu indicates that the computer is maintained by an educational institution. Country codes also serve as top-level domains. Canada’s top-level domain is .ca, the United Kingdom’s is .uk, Australia’s is .au, and the European Union uses .eu as a top-level domain. Historically, businesses used the .com domain, while nonprofit organizations used .org. Educational institutions used .edu. The .net domain was usually used by communication companies. Businesses generally obtained domain names with all applicable top-level domains (e.g., nike.com, nike.org, and nike.net) to prevent their use by competitors or fake businesses. Businesses also tried to acquire domain names with similar sounding names or those that are common misspellings of their corporate names. Today, there are hundreds of additional top-level domains, such as .biz, .co, and .fit. Obtaining all the TLDs for a business name is not practical. Even within a sector such as education, .edu has been joined by .academy, .education, .guru, .institute, .training, and .university.
.◗How does the domain name system work? Scattered around the world are several domain name servers that maintain lists of all domain names and their corresponding IP addresses. In addition, frequently used domain names are stored by ISPs, educational institutions, organizations, and Internet companies such as Google. When new domain names are added, these lists are updated. New domain names may take several days to propagate to all of the lists, which is why new Web sites can be accessed only by their IP addresses for the first few days they are in operation. A Web site can be accessed by its domain name once the name is added to the domain name server lists. Suppose you want to look at the newest selection of Nike athletic shoes. You enter nike.com in your browser’s address bar. Your browser must find the IP address that corresponds to nike.com before it can connect you to the site. Your browser queries a domain name server to get the IP address, a process that generally happens without noticeable delay (Figure 3-26).

◗So what makes the DNS one of the lnternet's weaknesses?
QUlCKCHECK
Altering the DNS records can change the destination of email, browser con- . . Each record in the domain name nectlons, and download requests. Unauthorlzed changes to the DNS are system called DNS spoofing. a. represents a correspondence between a domain name and an IP address Hackers use DNS spoofing so that queries for legitimate sites are directed to fake sites. Some governments use DNS spoofing to redirect searches from culturally or politically inappropriate sites to government-approved sites. Figure 3—27 illustrates what happens when unauthorized changes are b- pſOteCtS domain names from made in the DNS. DNS SpOOfing c. routes packets over the
◗Can domain name servers be turned off? Yes, though more com- TRY lT! monly a DNS server operated by your lSP will go offline as a result of an equipment failure. When the DNS you're using goes down, the process of accessing the lnternet can get very slow while DNS requests are routed to make sure it hasn't been through alternate servers. it is even possible that a DNS outage can leave changed by a hacker. What is you without lnternet access unless you enter raw lP addresses. the lP address of your DNS? is For security, you should periodicallv check your DNS it is a good idea to know how to find your DNS server and how to change it a private address residing on it in case of an outage. Figure 3—28 demonstrates how to find your DNS vour router or a public address? settings.

◗What can ! use as an alternative DNS server? Surprisingly, you QUlCKCHECK might be able to bypass DNS outages, blocks, and hacks by changing your DNS server. Errors in the domain name tables may be limited to the domain name server that you are using. Connecting to a different domain name When might you want to change your DNS server? server might restore full access. ln addition, when you are traveling—espe- a. When the DNS server that l'm cially overseas—a local DNS server might be closer and quicker. using goes down Keep the numbers in Figure 3-29 handy in case you need to change your b. When l'm traveling and other domain name server. Write them down, or store them locally on your device. DNS servers are closer Remember, if your domain name server has an outage, you won't be able to use domain names to search the lnternet for a solution.




CONNECTION BASICS
With online access becoming an essential component in our daily lives, sociologists have been tracking Internet “haves” and “have-nots.” According to the Pew Research Center, an estimated 80% of American adults have Internet access. Worldwide, about 40% of the population has Internet access. Not all of these connections are fast and dependable. Let’s take a look at what makes a good Internet connection.
◗How fast is the Internet? Data travels over the Internet at an incred- ible speed, but that speed varies. Some Internet services are faster than others. Speed is affected by the amount of traffic flowing over the connec- tion. It slows down when usage is heavy. Slowdowns also occur as a result of faulty routers and when hackers launch denial-of-service attacks that overwhelm server capacity.
It is easy to check the speed of your Internet connection by running a few online tests. Figure 3-30 shows results from testing speed in a rural area. Is this connection suitable for streaming movies, playing online games, and conducting video chats?


◗ What does connection speed measure? Speed is the distance something travels in a specified amount of time. The speed of a car, for example, is measured in miles (distance) per hour (time). What we casually refer to as “connection speed” has little to do with distance. The most com- mon measurement of connection speed is the amount of data that can be transmitted in a specified time. Technically, it is a measure of capacity. But let’s use nontechnical terms and call it speed.
The speed test in Figure 3-30 produced a download speed of 46.42 Mbps because it was able to move 46 megabits of data down from a server to the user’s computer in one second. The upload speed at which data is transferred from the computer to a server was only 24.27 Mbps. With this connection, downloading a two-hour movie from iTunes would take about 4 minutes. Uploading a 4 MB photo would take less than a second.
QUICKCHECK
Would the connection used for
the speed test in Figure 3-30 be classified as broadband?

1. Yes, it is synchronous.
2. Yes, the download speed
3. as broadband.
4. No, its minimum speed
5. 17.22 Mbps is not fast
7. No, its average speed is only
   24.27 Mbps.  

◗How much speed do I need? For email, browsing the Web, and streaming videos, 0.5 Mbps (500 Kbps) speeds are adequate. However, other activities require higher speeds, as shown in Figure 3-31.


◗ Why are upload and download speeds different? ISPs control connection speeds based on the service plan you’ve selected. Your band- width cap is the top speed allowed by your plan. During peak times, ISPs can place further limits on speed, a process called bandwidth throttling.
When Internet upload speed differs from download speed, you have an asymmetric connection. When upload and download speeds are the same, you have a symmetric connection.
Most Internet connections are asymmetrical, with upload speeds consid- erably less than download speeds. Asymmetric connections discourage subscribers from setting up Web and email servers that would transmit lots of outgoing data. For most users, however, an asymmetric connection is sufficient, but download speeds of less than 1.5 Mbps may not provide the full Internet experience.
◗ What is Ping? Ping is utility software designed to measure respon- siveness. Ping rate indicates how quickly data can reach a server and bounce back to you. Ping was named after the sound that a submarine’s sonar makes when it bounces off an undersea object.
Technically, Ping measures latency. Latency is the elapsed time for data to make a round trip from point A to point B and back to point A. Latency is measured in milliseconds (ms). A millisecond is a thousandth of a second, so data transmitted on an Internet connection with 100 ms latency makes a round trip in one-tenth of a second.
QUICKCHECK
In the table at left, why are
there recommended upload
speeds only for Skype?

1. Skype doesn’t use uploads.
2. Skype requires two-way
   communication, whereas

the other services use most
of the bandwidth only for

2. Skype is the slowest service
3. those listed.
4. Amazon, YouTube, and Netflix
5. different levels of

service, so the upload speed
cannot be specified.
Terminology
Download and upload speeds are sometimes referred to
as downstream speeds and
upstream speeds, respectively.  
◗ What else affects the speed of my connection? Your Internet connection can be affected by outages and traffic. In Section B, you found
out what can happen when a DNS server goes down. Service disrup- tions can also occur at ISPs, cloud storage sites, email servers, and other Internet-based services. You can use online tools such as Akamai’s Real- time Web Monitor and downrightnow.com to check the status of various Internet services.
◗ What are my Internet connection options? Although public Internet access is available in many locations, such as coffee shops and libraries, most consumers like the convenience of having their own Internet connection. Depending on your geographical location, you may have several options for connecting to the Internet. Before we look at the most popular Internet access options such as cable Internet service and cellu- lar broadband, consider the pros and cons of the three classifications of Internet connections in Figure 3-34.

Try iT!
Are any service disruptions
occurring right now? Connect to downrightnow.com to find out.
QUICKCHECK
A local ISP advertises a modem
that you can carry to any
location and plug in to access
the Internet. This device would be used for __ Internet access.

1. fixed
2. portable
3. mobile
4. none of the above
   ◗Are there different kinds of LaNs? LANs can be classified by their protocols; Ethernet and Wi-Fi are the two most popular. The Windows oper- ating system includes a tool for setting up a LAN called a homegroup that makes it easy to share files among local computers, but does not provide

Internet access. MacOS also provides a tool called AirDrop for making a point-to-point connection between two computers. Most LANs, however, are set up using a router so that they have proper security and Internet access.
◗Are LaNs regulated by the government? Most wireless LANs use the 2.4 GHz and 5.0 GHz unlicensed frequencies so that they can be set up without applying to the FCC for permission. The few unlicensed fre- quencies are crowded, however, and neighboring networks that are forced to use the same frequencies pose security risks.
◗Are my devices equipped to access LaNs? The circuitry that enables a device to access a LAN is called a network interface controller (NIC). NICs are built into the circuit boards of most digital devices. NICs are also available as add-on circuit boards and USB devices.
NICs contain a MAC address (media access control address) used to uniquely identify devices on LANs. MAC addresses are usually assigned by the manufacturer of digital devices and embedded in the hardware.
A MAC address functions in conjunction with an IP address on a LAN. Each device on a LAN has a MAC address (sometimes listed as the Wi-Fi address or physical address). DHCP assigns an IP address to a device and links it to the device’s MAC address. Figure 3-48 illustrates how to find the MAC address on various devices.

QUICKCHECK
Which frequencies does the FCC
not regulate?

1. Wireless frequencies
2. Radio frequencies
3. Internet frequencies
4. 2.4 GHz and 5 GHz
   Terminology

The term MAC address has
nothing to do with Apple’s
Mac computers. Both PCs and Macs have MAC addresses, as do smartphones, routers, and other data communication

1. Try iT!

Find the MAC address for the
device you’re currently using. Does it look similar to an IP address?  





wi-Fi
Wi-Fi refers to a set of wireless networking technologies defined by IEEE 802.11 standards. A Wi-Fi device transmits data as radio waves and is compatible with Ethernet, so you can use the two technologies in a single network.
◗How does Wi-Fi work? You can set up Wi-Fi in two ways. One option is to use wireless mesh topology in which devices broadcast directly to each other (Figure 3-53).

A second option for Wi-Fi networks is a star topology in which a centralized broadcasting device—a wireless access point—coordinates communication among network devices. Technically, the centralized device is a wireless access point, but that function is built into most routers (Figure 3-54).

Terminology
Wireless mesh networks
are sometimes called peer
networks or ad-hoc networks. Wireless networks that depend on a router are sometimes
called wireless infrastructure
networks or managed

1. QUICKCHECK

Which Wi-Fi infrastructure is
most similar to Ethernet?

1. Wireless ad-hoc
2. Wireless infrastructure  
   

FILE SHARING BASICS
Networks provide access to all types of files. From Snapchats to feature- length movies, from magazine articles to full-length novels, and from sound effects to symphonies, a wide variety of files exist on local networks and on the Internet.
You may spend lots of time downloading files to your local devices, but there are also times when you want to provide access to files you have stored locally or in the cloud. File sharing makes the magic happen.
◗ What is file sharing? File sharing allows files containing docu- ments, photos, music, and more to be accessed from computers other than the one on which they are stored. Sharing can take place within a LAN or across multiple networks, including the Internet.
◗How does file sharing work? When computers connect to other computers on a network, their users may be able to view a list of files stored on the remote device. Given permission, users can open, view, edit, copy, and delete files from the remote device.
◗Are there restrictions on file sharing? Your ability to share files with other devices on a network depends on several factors, which are listed in Figure 3-67.

◗ What if other network devices aren’t listed? The network utilities provided by operating systems such as Windows and macOS automatically detect other devices when network discovery is turned on.
Network discovery is a setting that affects whether your computer can see other devices on a network, and whether your computer can be seen by oth- ers.
When network discovery is turned on, other computers that connect to your local area network display the name of your computer in the list of network devices. When network discovery is off, the name of your computer will not appear in the list of network devices. Network discovery works in different ways on different devices.
Mobile devices. The operating systems on mobile devices may not offer a way to see other devices on a network or to broadcast their presence on a network.
Macs. MacOS devices, such as iMacs, have no user-modifiable network discovery settings, but offer file sharing settings instead. If file sharing is On, then network discovery is enabled.
Windows. Some operating systems, such as Windows 10, offer a network discovery setting that allows users to turn network discovery off or on. When using public networks, this setting should be off (Figure 3-69).