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Computer Networks
Transport Protocols & IP Addressing
GATE 2015
Medium

Which one of the following fields of an IP header is NOT modified by a typical IP router?

  • AChecksum
  • Source address
  • CTime to Live (TTL)
  • DLength

Explanation

A router decrements the TTL field on every hop, which forces the checksum to be recomputed, and the Length field can change if the packet is fragmented. The source address, however, is left untouched by standard (non-NAT) IP routing — only the destination MAC/next-hop routing decision changes, not the IP source address.

Source: GATE CS 2015 (Session 1 / 7th Feb Shift 1) (2015)

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