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?
Official answer and solution
- Official answer
- A and C
- From
- Official GATE 2024 answer key
Solution
Identify which IPv4 header fields change when an outbound packet's private source IP address is translated to a public IP address by a NAT router.
When an outbound IP packet passes through a NAT device from an internal (private) network to an external (public) network:
- (A) Source IP is modified: The private Source IP address is replaced by the NAT router's public IP address.
- (B) Destination IP is not modified: The external destination server's IP remains the same.
- (C) Header Checksum is modified: Because the IPv4 Header Checksum covers the entire IPv4 header (including Source IP), modifying the Source IP requires recomputing and updating the Header Checksum.
- (D) Total Length is not modified: Replacing a 32-bit IPv4 address (and 16-bit TCP/UDP port) does not change the packet's length.
Options (A), (C)
Forgetting that modifying the Source IP address in the IPv4 header invalidates the existing IPv4 Header Checksum.
The answer is the one in the official GATE answer key. The solution is this site's own explanation.
Source
- Paper
- GATE 2024 Computer Science and Information Technology - Set 1 (CS1)
- Question
- 31 of 65
- Section
- Computer Science and Information Technology
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