Final answer:
The TTL field in an IP packet header prevents packets from circulating indefinitely and specifies the maximum number of hops a packet can take. Routers decrement the TTL value and discard packets that have reached zero. This prevents network congestion and ensures timely packet delivery.
Step-by-step explanation:
The TTL (Time to Live) field in an IP packet header is used to prevent packets from circulating indefinitely in the network. It specifies the maximum number of hops (routers) a packet can take before being discarded.
When a router receives a packet, it decrements the TTL value by one. If the TTL value reaches zero, the router discards the packet and sends an ICMP (Internet Control Message Protocol) message back to the sender indicating that the TTL exceeded.
By discarding packets that have exceeded the TTL value, routers prevent network congestion and ensure that packets reach their destination within a reasonable time frame.