Which of the following fields is/are modified in the IP header of a packet going out of a network address translation (NAT) device from an internal network to an external network?
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GATE CSE 2024 Set 1 · Q31
MSQ1 markEasy
Time
Which of the following fields is/are modified in the IP header of a packet going out of a network address translation (NAT) device from an internal network to an external network?
Select all that applyOne or more correct
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Solution
Step-by-step verified solution
Official answer: A) Source IP; C) Header Checksum
Network Address TranslationWhen an IP packet travels from an internal (private) network to an external (public) network through a NAT router:
Source IP Address: The private source IP address of the internal host is translated (replaced) with a globally routable public IP address assigned to the NAT device. Thus, the Source IP is modified.
Destination IP Address: The destination IP address remains that of the external server/host and is not modified for outbound packets.
Header Checksum: Because the Source IP address inside the IPv4 header has changed, the IPv4 Header Checksum must be recalculated and updated.
Total Length: Translating an IPv4 address does not change the size of the header or the payload, so the Total Length remains unchanged.
Therefore, the correct options are A and C.
Try the question first, then check your work.
Examiner trap
The misconception this question tests
Recall that outbound NAT replaces private local addresses with a public address and requires recomputing the checksum.
This Network Address Translation previous year question is Q31 of the GATE CSE 2024 Set 1 paper. Find more GATE CSE PYQs on Network Address Translation in Computer Networks, or browse all GATE CSE PYQs.
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Network Address Translation in GATE CSE
GATE CSE 2024 Set 1 Q31 is the only Network Address Translation question in the GATE CSE papers so far.
How GATE tests Network Address TranslationMore from GATE CSE 2024 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.