A+ Question of the Week: Networking Software Tools

The correct answer is C.

The traceroute function manipulates the time-to-live (TTL) field of the IP header. It sends a ping packet out with a TTL of 1, which is guaranteed to trigger a “TTL exceeded in transit” ICMP message from the first router back to the device that originated the trace (ran the TRACERT command). The identity of the router is learned from the source of this message.

Subsequent pings are sent out with the TTL incremented by 1 each time. This guarantees passage through the previous router and that the next router in the path to the destination specified in the TRACERT command will be the one to execute the ping and send back an ICMP message identifying itself as the executioner.

The trace ends when the destination host (the one specified in the TRACERT command) responds to the ping, letting the originating host know that the traced path to the destination is complete. With this baseline created, subsequent traces (made during a network outage and compared to the baseline) have a chance of identifying the point in the path where the outage has occurred. This is not guaranteed when there are multiple paths through the network between the same source and destination hosts.