Which one of the following fields of an IP header is NOT modified by a typical IP router?
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GATE CSE 2015 Set 1 · Q22
MCQ1 markEasy
Time
Which one of the following fields of an IP header is NOT modified by a typical IP router?
Select one answerSingle correct
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Solution
Step-by-step verified solution
Official answer: B) Source address
IPv4, CIDR and NATWhen a typical (non-NAT) IP router forwards an IP packet:
Time to Live (TTL): The router decrements the TTL by at least at each hop to prevent routing loops.
Checksum: Since the TTL field changes, the IPv4 header checksum must be recomputed at every router.
Total Length: If the packet exceeds the MTU of the outgoing interface and fragmentation occurs, the Length field in the fragmented headers is modified.
Source Address: A standard IP router preserves the original Source IP Address and does not modify it (unless it is explicitly performing Network Address Translation, NAT).
Therefore, the Source address is NOT modified by a typical IP router, making (B) the correct answer.
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Examiner trap
The misconception this question tests
Consider which IP header fields must change at every hop (like hop limit and integrity check) versus those identifying the originating host.
This IPv4, CIDR and NAT previous year question is Q22 of the GATE CSE 2015 Set 1 paper. Find more GATE CSE PYQs on IPv4, CIDR and NAT in Computer Networks, or browse all GATE CSE PYQs.
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IPv4, CIDR and NAT in GATE CSE
IPv4, CIDR and NAT has 14 GATE CSE PYQs across 10 papers from 2012 to 2025. 5 of them are 1-mark MCQs like this one, and it is the only one in GATE CSE 2015 Set 1.
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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.