Traceroute is a network diagnostic utility that discovers the path packets take between your computer and a destination host, revealing the IP address and round-trip latency of each intermediate router (hop) along the way.
How it works
Traceroute sends a series of probe packets with progressively increasing IP Time-To-Live (TTL) values, starting at 1. Each router along the path decrements the TTL by 1; when TTL reaches zero, that router discards the packet and sends back an ICMP "Time Exceeded" message, revealing its IP address. By incrementing the TTL (1, 2, 3...), traceroute discovers each successive hop. The destination responds differently—Unix/Linux versions typically send UDP packets to an unlikely port, triggering an ICMP "Destination Unreachable" response; Windows tracert and others use ICMP Echo Request/Reply instead.
Example
$ traceroute example.com
1 192.168.1.1 1.123 ms
2 10.10.0.1 8.431 ms
3 203.0.113.5 15.220 ms
4 93.184.216.34 22.904 ms (destination)
Each row shows a hop; typically three probes per TTL measure round-trip times.
When it matters
Traceroute is essential for diagnosing routing problems, latency issues, and understanding network topology. It helps identify where packets are being delayed or dropped and whether a target is reachable at all.
WarningMany routers and firewalls rate-limit or block ICMP/UDP probes, causing traceroute to show
* * *(timeouts) even when the route beyond that hop works fine—this is often mistaken for an outage.