From Tibbo Technology:
AggreGate LinkServer for Linux lets both sides of incoming connection links to communicate through a "middle-man" (Link Service), to which they both can establish outbound connections. When two people use a messenger, they connect to a central server. Neither of the users typically has a static IP address, but the server has a static IP. Both parties make outbound connections, so no firewalls have to be configured and neither side needs a static IP.