Which of the following fields is/are modified in the IP header of a packet…
2024
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?
Answer: A. Source IP; C. Header Checksum — Answer: Source IP and Header Checksum Source IP: NAT rewrites the packet's internal source address to the public/mapped address for outbound traffic, so the…
- A.
Source IP
- B.
Destination IP
- C.
Header Checksum
- D.
Total Length
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Correct answer: A, C
Answer: Source IP and Header Checksum
Source IP: NAT rewrites the packet's internal source address to the public/mapped address for outbound traffic, so the Source IP field in the IP header is changed.
Header Checksum: Because the IP header is modified, the IPv4 header checksum must be recalculated. (IPv6 does not include an IPv4-style header checksum.)
Destination IP: Not changed by typical outbound NAT — the packet keeps the original destination address.
Total Length: Remains the same unless NAT also changes payload size (for example, fragmentation or payload modification). Standard NAT rewrites addresses but does not change the overall IP packet length.
Quick note: Transport-layer port mappings (e.g., TCP/UDP ports) are performed in the transport header, not the IP header, so they are not part of this question.