The GATE Grind

GATE 2015 CS – Question 26

Computer Networks · IPv4: Addressing, CIDR, Fragmentation and NAT · 1 mark · Multiple choice

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

  1. Checksum
  2. Source address
  3. Time to Live (TTL)
  4. Length

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Show answer and explanation

Correct answer: (B) Source address

Explanation

A router lowers the TTL at every hop, so the checksum has to be recomputed, and the length changes when a packet is fragmented. The source address stays the same all the way to the destination, except in the special case of NAT.