time to live (TTL)

TTL data field in the IP header

The TTL field is a single byte field in the IP header, which restricts the lifetime of datagrams. The counter is located within the IP header and is decremented by each router. The datagram is destroyed when the value drops to "0". The unit of time is a second. The relative lifetime of a datagram is 255 seconds at the most, or the datagram can travel across 255 hops.