Traceroute

Traceroute - - finds the route packets take between you and the selected address

Traceroute isn't foolproof. Some networks block ICMP packets at their boundaries, making it impossible to run traceroute into their systems.

Traceroute will send three packets for each hop of the route. Each packet that doesn't return is shown as an asterisk. If none of the three packets return from a stage it will be shown as * * * failed

If a single stage fails, but later stages return data it means the one stage doesn't respond to traceroute packets

If traceroute returns data up to a stage, but all stages after that fail it usually means that the stage after the final one is a firewall blocking ICMP packets. This is... suspicious.

A few older routers are faulty in the way they handle traceroute packets. They will set the lifetime of the error packets they return to the same lifetime as the packet they receive. This is pretty rare (I've never seen it) but the symptom is that stages 1..n-1 will return traceroute packets, stages n..2*n-1 will fail and stage 2*n will return the packet that should have appeared in stage n

Another description of traceroute