Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?
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GATE CSE 2024 Set 2 · Q32
MSQ1 markEasy
Time
Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?
Select all that applyOne or more correct
Pick every correct option
Solution
Step-by-step verified solution
Official answer: C) Time to Live (TTL); D) Header Checksum
IPv4, CIDR and NATWhen an IPv4 router forwards a packet:
Time to Live (TTL): The router decrements the TTL field by at least 1 to prevent packets from looping endlessly in the network.
Header Checksum: Because the TTL field (and potentially options) is modified, the router must recompute and update the 16-bit IPv4 header checksum.
Other fields:
Source IP Address: Remains unchanged unless Network Address Translation (NAT) is explicitly being performed.
Protocol: Specifies the higher-level transport protocol (e.g., TCP, UDP) and remains invariant throughout the transmission path.
Therefore, options C and D are correct.
Try the question first, then check your work.
Examiner trap
The misconception this question tests
Recall which header fields must change at every hop to prevent infinite loops and maintain data integrity of the header.
This IPv4, CIDR and NAT previous year question is Q32 of the GATE CSE 2024 Set 2 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. This is the only 1-mark MSQ among them, and it is the only one in GATE CSE 2024 Set 2.
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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.