TTL (Time To Live)

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 (28-1, 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

74570c20.qxd 9/12/07 12:44 AM Page 502

• NIC (Network Interface Card)
• NID (Network Interface Device)
• Nigerian Connection
• Nigerian Scam
• NII (National Information Infrastructure)
• NIR (National Internet Registry (NIR)
• N-ISDN (Narrowband ISDN)
• NIST
• NIST (National Institute of Standards and Technology)
• NIU (Network Interface Unit)
• NLOS (Non-Line-Of-Sight)
• nm (nanometer)
• NMS (Network Management System)
• NMT 450 (Nordic Mobile Telephone 450 MHz)
• NMT 450 (Nordic Mobile Telephone 450 MHz)
• NMT 900 (Nordic Mobile Telephone 900 MHz)
• NMT 900 (Nordic Mobile Telephone 900 MHz)
• NMT (Nordic Mobile Telephone)
• n (nano-)
• NNI (Network-to-Network Interface)
• NN (Network Neutrality)
• NOC (Network Operations Center)
• node
• noise
• nominal