Traceroute
Traceroute is most commonly used to troubleshoot connectivity issues. If all you know is that you can't get to host D from host A, traceroute will show you whether the connectivity loss exists at one of the intermediate routers—B or C or elsewhere. Note that traceroute works at Layer 3 and is most commonly implemented for IP using UDP.
In the first set of packets sent, the time-to-live (TTL) field is set to 1 and the port number is set to a port that is not likely to be valid, most commonly 33434. The consequence of setting TTL to 1 is that the first node receiving this packet will decrement the TTL, notice that the TTL is now 0, drop the packet, and return an ICMP timeout message. The TTL and port numbers are increased by one for ...
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