A common problem that some customers encounter when using a NAT router is that they want to be able to access their web site from within their local network, but their NAT router does not support “loopback” connections. This prevents them from being able to reach their own services, even though external visitors are able to connect.
Let’s say, for example, that we have a local network consisting of two machines behind a NAT router with the router in turn connected to a DSL or Cable modem. Each of the two machines will be assigned an IP address by the NAT router within a “local” IP space such as the 192.168 block. The NAT router in turn is assigned an IP address by the ISP within the “public” space. When you setup a host or domain within our system, it is this public IP address that the host/domain points to.
The following diagram shows the example network’s two computers behind a NAT router:
In this diagram, the “public” address is 188.8.131.52 and the NAT router has a LAN/”private” address of 192.168.0.1. The other computers on the network are assigned numbers within this private space.
Now, let’s say that we have registered the domain “example.com” with the host “www.example.com” mapped to this public IP address. When somebody out on the internet attempts to connect to www.example.com, they end up connecting to the WAN interface on the NAT router. The NAT router in turn “routes” the packets to an internal machine based on the port that the connection came in on.
The loopback problem arises when somebody using the “Work Station” computer wants to access the “Web Server” computer. If they try to access the computer via the hostname www.example.com, the NAT router will attempt to route this out its WAN interface and then back in. In most cases this will either fail or return the web interface for the router itself because the router doesn’t understand what the user is trying to do.
The Answer for: loopback_connections