Which field in the IP header indicates the maximum time a packet may remain on the network?

• A. TCP Window Size
• B. IP Header Checksum
• C. Protocol Number
• D. TTL Field

Answer: (D)

This item tests how IP controls how long a packet can live on the network. The field is the Time To Live (TTL). It sets the maximum number of hops a packet can take; each router that forwards the packet decreases TTL by one. If TTL reaches zero, the packet is discarded to prevent looping, and an ICMP Time Exceeded message may be returned. This is different from the TCP Window Size, which is used for flow control at the transport layer; the IP header checksum, which verifies header integrity; and the Protocol Number, which identifies the next protocol in the payload. TTL acts as a hop limit to bound a packet’s lifetime in the network and, in practice, helps prevent endless circulation.