The simplest way to verify OSI Layer 3 connectivity between two routers is to use the Internet Control Message Protocol (ICMP). ICMP defines a number of message types in RFC 792 for IPv4 and RFC 4443 for IPv6. (For copies, go to .) ICMP defines procedures for echo (ping), traceroute, and source notification of unreachable networks. Pinging an IP address can result in a variety of ICMP messages, but the only message indicating that a ping is successful is the ICMP echo reply message indicated by an exclamation point (!) in the output of the ping command
