With Subversion, it's defintely using a port number.
In any case, I have a domain name pointing to server.mysite.com for example. When I am outside the network where the server resides, I can access server.mysite.com with no problem. If I am attached to the network where the server resides, I get prompted to enter user name and password by the router. The port number can be any of the ones I have forwarded, and it will prompt me.
I have been through serveral routers, and this is the first time I have had this problem. I am cheating right now my controlling the name resolution via the hosts file. It's a pain because I have to change when I move in and out of the network.