Server to server communication over NAT/router

I have two servers that need to be able to send requests to each other开发者_运维问答, and I need them to be able to communicate over a NAT or router. One server has a registered domain, and it is always waiting for connections. The other server sends the first request (the login request) to the first server when it starts. What is the best way to allow the two server to continue to communicate?

Huh? This shouldn't be a problem. If the first server, the one with the registered name, is always contacted by the other one.

As long as the second machine has general Internet connectivity, all it needs to do is e.g. open a TCP/IP connection to firstserver.example.com or whatever, and they should be able to continue to communicate over that connection indefinitely.

This is no different from general surfing using NAT.