icmp的报文，
Destination Host Unreachable的意思如下：
The error message
"Destination Host Unreachable " tells that the ping request from our computer cannot find the route to the destination IP address(destination network device). It means the Packet send from your computer reached the destination network successfully but it failed to find the destination host. Destination host can be any networking device (with one unique IP address) in the destination network. One major possibility for this error is there is no route for the packet send from your computer to destination host. If there is no route available, it is impossible to find the destination host and your computer will get an error message "Destination host unreachable" instead of replies. So the issue can be due to faulty routing table. If the destination host is down at the time the packet send, it may result destination host unreachable error message. So here I conclude all possible reasons for Destination host unreachable error message with possible troubleshooting steps.
THE POSSIBLE REASONS FOR THE ERROR DESTINATION HOST UNREACHABLE
1. The ping packet find the destination network but failed to find the destination computer in that network.
2.Packet Routing issue.
3. Destination host might be down
http://blog.csdn.net/commshare/article/details/6591063
如果ping 127.0.0.1和ping 主机名可以，则很可以是网线没有插好。譬如插错网口了（双网卡的情况）
此时使用traceroute命令时，结果中会有!H
