1.1.d TTL

from routing tcpip vol 1…

If the router is told to trace the route to a host address such as 10.11.12.13, the router will

send three packets with the TTL set to one; the first router will decrement it to zero, drop the
packets, and send back error messages to the source. By reading the source address of the
error messages, the first router on the path is now known. The next three packets will be
sent with a TTL of two. The first router decrements to one, the second to zero, and an error
message is received from the second router. The third set has a TTL of three, and so forth,
until the destination is found. All routers along the network path will have identified
themselves.