DDNS relayThe forwarding of IP packets to an alternate server port by a DDNS provider. Residential customers and many small businesses typically use an entry-level Internet service that may not allow them to host a Web server to the world at large. ISPs can restrict these customers by blocking all traffic destined to port 80, which is the common TCP/IP port number assigned to a Web server.
Customers can circumvent this restriction by assigning a different port number to their Web servers and moving their DNS authorities to a DDNS relay provider (see DDNS). The DDNS provider looks for all requests to the customer's "www" or other designated subdomain, changes port 80 to the new port number and forwards the packets. See TCP/IP port and well-known port.