Networking 101: Understanding (and Using) ICMP

As Networking 101 begins moving on up the stack toward the layers involved with routing, we must pause for a moment. Some attention needs to be paid to the most misunderstood protocol: ICMP. Managers and administrators alike should understand what ICMP is really used for if they plan on making firewall policy decisions, and administrators can use ICMP knowledge to fully understand routing issues.

Link: Enterprise Networking Planet