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Time to live (TTL) is a mechanism used to limit the lifespan of data on a network. Data is discarded if the prescribed TTL elapses. The idea behind having a TTL is to prevent any data packet from circulating indefinitely.
Time to live (TTL) is basically the number of hops that a packet travels before being discarded by a router. Specific TTL numbers indicate the maximum range for packets.
The initial TTL value is set as an eight binary digit field of the packet header by the sending host. The TTL field is set by the sender of the datagram and reduced by every router on the route to its destination. When forwarding IP packets, the router decreases the TTL value by at least 1. When the packet TTL value reaches 0, the router discards it and sends an Internet Control Message Protocol (ICMP) message back to the originating host.