GATE CS 2024 Set 1 — Question 31

MSQ+1 / -0MediumNATNetwork Layer: AddressingComputer Networks

Computer Networks → Network Layer: Addressing → NAT

Last updated 20 August 2026

Question

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?

Correct answer (A) Source IP; (C) Header Checksum Solution When a packet goes from an internal network to an external network through a NAT device (specifically Source NAT or NAPT): 1. Source IP : The NAT device replaces the private source IP address of the internal host with its own public IP address (or one from a pool). Thus, the Source IP is modified. 2. Header Checksum : Since the IP header contains the Source IP address, changing the Source IP invalidates the existing checksum. The NAT device must recalculate and update the Header Checksum. 3. Destination IP : For an outgoing packet, the destination is an external host, so the Destination IP is typically not modified (unless it is a specific destination NAT scenario, which is not the default for internal-to-external traffic). 4. Total Length : The size of the packet usually remains the same, so the Total Length field is not modified. Therefore, the fields modified are Source IP and Header Checksum. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (A) Source IP; (C) Header Checksum Solution When a packet goes from an internal network to an external network through a NAT device (specifically Source NAT or NAPT): 1. Source IP : The NAT device replaces the private source IP address of the internal host with its own public IP address (or one from a pool). Thus, the Source IP is modified. 2. Header Checksum : Since the IP header contains the Source IP address, changing the Source IP invalidates the existing checksum. The NAT device must recalculate and update the Header Checksum. 3. Destination IP : For an outgoing packet, the destination is an external host, so the Destination IP is typically not modified (unless it is a specific destination NAT scenario, which is not the default for internal-to-external traffic). 4. Total Length : The size of the packet usually remains the same, so the Total Length field is not modified. Therefore, the fields modified are Source IP and Header Checksum.

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