The packet leaving the NAT router has its source IP address replaced with the router's public IP, and the destination IP remains unchanged (or modified based on port forwarding). Source port may be modified, while the destination port typically stays the same.
When a NAT router processes an IP packet with a private source IP address and a private destination IP address, along with a TCP payload carrying a source port number and a destination port number, the NAT router modifies the packet as follows:
1. Source IP Address:
- The NAT router replaces the private source IP address with its own public IP address.
- The source port number may be modified as well.
2. Destination IP Address:
- The NAT router leaves the private destination IP address unchanged unless it is configured with port forwarding rules.
- If there are port forwarding rules, the router may replace the private destination IP with the corresponding public IP from the port forwarding rule.
In summary, the packet leaves the NAT router with:
- The NAT router's public IP address as the source IP address.
- The original destination IP address (or modified based on port forwarding rules) as the destination IP address.
- The source port number may be modified.
- The destination port number remains the same unless there are specific port forwarding rules.