Seeing The Route
Ping tells you if a host is reachable, but not the path taken. The traceroute tool reveals each router, or hop, along the way to a destination.
The Time To Live Trick
Traceroute cleverly abuses a field meant to stop loops.
- Every packet has a time to live value that drops by one at each hop.
- When the value hits zero, that router discards the packet and sends back a notice.
- Traceroute sends probes with increasing time to live, so each one dies one hop further along.
The router that drops a probe reveals its own address, so the tool builds the path one hop at a time.
Reading The Hops
Each line shows a hop number, the router address, and a few timing samples. A row of asterisks means that hop did not reply, which can be a firewall hiding it rather than a real break. Rising times deep in the path can hint at where latency is added.
Key idea
Traceroute sends probes with increasing time to live so each one expires a hop further along, revealing the routers on the path, though asterisks may mean a hop hides itself rather than a real failure.