Traceroute
The traceroute utility also uses ICMP messages. This utility shows all the routers along the path between two points. You can use it to determine where communications are breaking down along a particular path.
The first column in the output of traceroute on the next slide is a hop number, which is also the TTL value set in the packet.
Traceroute begins its process by sending out a packet with a TTL of 1. When the packet hits the first router, it returns a message that the TTL has expired.
Then, traceroute adds one to the TTL and sends another packet. That packets goes to the second router and is similarly returned. This process is repeated until the destination host is reached and all routers along the way are identified.