If the TTL reaches zero, the packet has expired and is discarded. By using small TTL values which quickly expire, traceroute causes routers along a packet's normal delivery path to generate these ICMP messages which identify the router. A single router should respond, using the IP address of the interface it transmits the ICMP Timeout messages on, which should be the same as the interface it received the original packets on. The user is told this IP address, and DNS is used to convert this into a symbolic domain address. Also, round trip times are reported for each packet in the group.
H means host unreachable, etc.
Changing paths Always remember - you are not tracing the path of one packet, but of many. Hopefully, all those packets will follow the same route, but this is by no means assured. Your packets may be rerouted, and traceroute's output becomes a confused combination of two separate routes. No sending addresses You only see one IP address from each router - the address closest to you. To put it another way, traceroute can't tell you which interfaces routers are sending the packets on.