The thing I find strange is when your internal computer has an external IP adress, which address is used to access the router. . if at all the thing is used and not bypassed while it's just functioning as one big switch.
In short, the WAN IP address should be unique on a global level. Using it for a WAN port will allows you to use reserved IP's on your own network while data get's send by an IP that is "allowed" to access the Net. If you
force that WAN IP to a single computer on the network, the router can not have that address [ that will conflict as there will be 2 computers with the same IP ] .. so which address get's used for the router ?
Besides that, your local network has a seperate IP range. Let's say it's 192.168.2.0. All computers on the internall net including the routers LAN port have a IP in that range. If not they can't send any data to eachother. So when you
force the WAN IP to a single local computer. . . . that computer is beyond the 192.168.2.0 range and will therefor not be able to communicate with the router nor any computer on your LAN.
Only option slightly like it I know of if using a secundairy IP address in the network settings of your adapter.
rigth clicking on your network adapter, select
properties and then the advanced options of the TCP/IP protocol. Second tab [ in Win2K ] shows the IP address and enables you to use a second IP for the adapter. Although a first IP is needed for that and it won't have any result behind a router as the router itself can't use 2 IP's for the LAN adapter.
Concluding, any router that enables the user to use the WAN IP address for any of the local LAN adapters should be returned I think. . . . ..sorry. .
[ again, verry short. . . you get only one IP of your ISP. If that one is given to an computer behind a router, then which IP is the router using so it can be accesed from that computer in your LAN ? ]
About it being OS dependant, your router has a seperate OS which handles all the settings for it. Internal IP's, WAN IP. . hosts, port block things like that. . . .you shouldn't be able to change those settings from an OS on your computer [ in the LAN ], that is a pretty big security flaw. You should only me able to configure the router while logged into the router. Not from any computer on the LAN.
And hence it should only be possible from the router, the most common options are to telnet or HTTP to the thing. Making it totally not OS dependant.
Those where ment as 2 seperate thing. Sorry for not making that more clear. Disabeling the firewall. And if that doesn't work place the computer in a DMZ.
Even more strange about Microsoft, MSN messenger was unable to transfer files from behind a Win2K router. That had been fixed with MSN 6. Their own software isn't even working with their own servers. . . .. besides that, ensuring they work behind a nat router as that would bypass Microsofts Windows. . well. . . .Win2K has the option of NAS routing in the ISA server .. . ..
Now for the most informatieve part.

I had to allowd some UDP ports which where all blocked by default on the router's firewall. That and some ports for
ntoskernel. If I wouldn't have done the first, connecting to online servers wasn't possible. If not the second joining a game prooved to be impossible.
