TTL

Problem

Suppose Router A sends a packet to B, B sends it to C, and because of a routing mistake C sends it back to A.

A → B → C
↑       ↓
└───────┘

Without intervention, that packet could circulate forever.

Solution — TTL

Every IP packet carries a counter. Common starting values are 64, 128, or 255, depending on the operating system and configuration of the device that sends the packet.

Each router decreases it:
64 → 63 → 62 → 61 ...

At zero, the router discards the packet.

Core idea: TTL prevents packets from living forever in a routing loop.