Basic ICMP Tools
Before you configure your network devices, you should learn how to use some ICMP (Internet Control Message Protocol) tools. ICMP packets help you root out problems with connectivity and routing.
ping (see http://ftp.arl.mil/∼mike/ping.html) is one of the most basic network debugging tools. It sends ICMP echo request packets to a host. If the host gets the packet and feels nice enough, it sends an ICMP echo response packet in return.
Let's say that you run ping 10.1.2.21 and you get this output:
PING 10.1.2.21 (10.1.2.21): 56 data bytes 64 bytes from 10.1.2.21: icmp_seq=0 ttl=255 time=8.0 ms 64 bytes from 10.1.2.21: icmp_seq=1 ttl=255 time=3.2 ms 64 bytes from 10.1.2.21: icmp_seq=2 ttl=255 time=3.4 ms 64 bytes from 10.1.2.21: icmp_seq=4 ...
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