The amount of time in seconds or router hops allowed before the datagram times out. Every router examines this value, decrements it by 1 and sends a time exceeded message to the source of the datagram also discarding the datagram when the TTL value reaches zero.

If a datagram is being sent a long distance through many routers it is sensible to pre-set a large TTL value so that the datagram does not expire on the way.