Some trace utilities, such as the IOS trace command, make use of the TTL field. 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...
How Traceroute works
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