Traceroute

Traceroute is a program for determining the path that packets take to reach another Internet host. It works by sending packets with short time-to-live (TTL) values, and seeing which hosts send back error messages. For example, a packet with a TTL of 1 will die at the first host along the route, and that host (if it is standards-compliant) will transmit an error packet back.

Traceroute is useful for debugging networking problems. If you aren't sure why you can't reach a particular computer, use traceroute and see where packet traffic stops, or starts to show high round-trip times.

Example
traceroute to www.heise.de (193.99.144.85), 30 hops max, 40 byte packets using UDP 1 192.168.0.1 (192.168.0.1)  0.312 ms   0.314 ms   0.290 ms  2  217.0.118.161 (217.0.118.161)  47.256 ms   50.184 ms   44.142 ms  3  87.186.248.66 (87.186.248.66)  43.035 ms   43.170 ms   43.043 ms  4  217.239.39.30 (217.239.39.30)  44.150 ms 217.239.39.26 (217.239.39.26)  44.056 ms   43.768 ms  5  217.243.218.38 (217.243.218.38)  45.033 ms   46.970 ms   44.201 ms  6  82.98.98.106 (82.98.98.106)  45.051 ms   43.964 ms   44.159 ms  7  82.98.98.106 (82.98.98.106)(N!)  45.003 ms * *
 * 1) traceroute www.heise.de