traceroute (was Re: Frustrating Intermittent Internet Connectivity Issues)
Traceroute works by sending UDP IP ICMP packets and occasionally incrementing TTL (time-to-live) field on the outbound packets. The intermediate routers at each "hop" which see these special packet have the option of replying to sender and when they do you will see their address and/or name and the associated latency/turnaround-time/delay from your computer to that router. When you see a traceroute output line with "* * *" it means that there is an intermediate router that is not responding to the traceroute ICMP packets due to configuration/rate-limit/filtering/management-decision. When traceroute program encounters one a not-responding-to-ICMP router along the path to destination address, it prints "*" and retries a few times and then increments the TTL and goes on to try to identify the next router in the path towards destination.