NAT Routing

Network Address Translation (NAT) is mapping one IP address space to another one by modification in transit.

Main Flow

1. Your server IP (10.0.1.5) sends a packet to the Internet.
2. Packet hits the NAT gateway; which sees its IP address belongs to the private ip address range.
3. NAT Gateway modifies the packet with its public IP. NAT Gateway logs to itself that 10.0.1.5 Port 50001 is NAT’s IP port 1001.
4. The Internet sees the packet now. And sends a packet back.
5. NAT Gateway sees the returned packet and checks its table. It can route it accordingly.

Note a connection (TCP/IP) is:

• Source IP
• Source Port
• Destination IP
• Destination Port
• Protocol

In an absolute sense, the client and ultimate server don’t know this switch has happened.