NAT Loopback allows you to access the external IP address from inside your home or office network. This is useful when you run a server inside your network. It allows you to use the same public IP address (and hence hostname) to access the server from both your home or office network as well as from the Internet.
In the example below, the Mail Server is located in the local network and it has a public IP address. It can be accessed from both the Internet and from the local LAN by using the public IP address (or Domain Name)
NAT Loopback is supported on all current router models running the latest firmware. It is enabled by default.