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Official PYQ · 2024Set 1 · Q21Fragmentation and IP Addressing, IPv4, CIDRMSQ1 mark

GATE CS 2024 Set 1 Q21 — Fragmentation and IP Addressing, IPv4, CIDR

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Which of the following fields is/are modified in the IP\text{IP} header of a packet going out of a network address translation (NAT)\text{(NAT)} device from an internal network to an external network?

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Answer: A;C

When an IP packet travels from an internal (private) network to an external (public) network through a Network Address Translation (NAT) device, the following modifications occur in the IP header:

  1. Source IP Address (Option A): The NAT device translates the private source IP address of the internal host to a public IP address. Thus, the Source IP is modified.
  2. Destination IP Address (Option B): The destination IP address remains unchanged because the packet is still destined for the same external host.
  3. Header Checksum (Option C): Since the Source IP address field in the IP header is modified, the Header Checksum must be recalculated and updated to ensure packet integrity.
  4. Total Length (Option D): The size of the IP packet does not change during address translation, so the Total Length field remains unchanged.

Therefore, the modified fields are Source IP and Header Checksum.

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