Overview
Time to Live (TTL) prevents packets from circulating indefinitely in a network loop. Despite its name, it is not a measure of time but a hop counter.
How it Works
Every router that processes a packet decrements the TTL value by 1. If the TTL reaches 0, the router discards the packet and sends an ICMP 'Time Exceeded' message back to the source.
Use in Diagnostics
Traceroute works by intentionally sending packets with increasing TTL values (1, 2, 3...) to identify every router along the path.