When a packet leaving the network undergoes Network Address Translation (NAT), which of the following is changed?

TCP sequence numbers are maintained end-to-end by the communicating hosts and are not modified by standard NAT processing.

NAT substitutes the private (RFC 1918) source IP address in outbound packets with the public IP address assigned to the NAT router or firewall, enabling internal hosts to communicate with external networks. The original private address and port mapping are stored in the NAT translation table so that return traffic can be correctly forwarded back to the originating internal host. This source address rewrite is the core mechanism that allows multiple private hosts to share a single public IP.

The destination port on outbound packets is not altered by standard NAT; source port reuse is a function of PAT/NAPT, and even then it is the source port, not the destination port, that may be reassigned.

The destination address identifies the external server the packet is sent to and is not modified by NAT on outbound packets - only the source address is rewritten.