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), (C)
Explanation
A NAT device lets hosts with private addresses share a public address. For a packet leaving the internal network it replaces the private source address with its own public address (with port translation it also changes the source port, but that is in the TCP or UDP header) and records the mapping so that replies can be sent back.
- Source IP: modified, as described.
- Destination IP: unchanged, because the packet still goes to the same external host.
- Header Checksum: modified. It covers the whole IP header, so it must be recomputed once the source address changes.
- Total Length: unchanged, because the sizes of the header and the data stay the same.
Answer: A, C