A field in the Internet Protocol version 4 (IPv4) header that specifies the length of time in seconds that the datagram can live in the Internet system.The maximum length of time is 255 seconds (2 8 -1, with 0 not considered, as it is the official time of death), or 4.25 minutes. From the instant the IP datagram enters the Internet, each gateway and host that acts on the datagram decrements the TTL by at least one second, although the time it has possession of the datagram generally is much less. When the TTL reaches 0, the datagram is declared dead and is discarded. The TTL mechanism prevents packets from wandering the Internet for eternity, at which point they would have no value, and would only contribute to overall network congestion. Over time, the TTL field has been redefined to indicate, as an option, the number of hops (i.e., routers) through which the packet travels. In effect, the TTL is a hop count, anyway.The default TTL is 64. See also datagram, gateway, header, host, Internet, IPv4, MPLS, and packet.