MAC Address filter?

Hey guys, this isn't a standard question, but I've been having a little trouble and I figured you guys would know better than anyone how to go about it.

Note:

I'm running OSX 10.4.11 on a Macbook Pro.

So, I'm trying to join a wireless network "ABC." It has a WEP key, but I've sussed it out, thanks to some tinkering and when I connect, my airport monitor and and AirRadar both register than I'm connected with nearly full signal strength. However, my computer has an auto-assigned IP - something in the 169.XXX.X.X range. I've tried to access the router directly, entering the host of 192.168.X.X addresses that usually correspond to router defaults, but I am unable to connect. My only thought is that the router is restricting IP or MAC addresses, so I tried guessing around, manually assigning myself IPs in the standard ranges - i.e. 192.168.1.7, but I realized that random guessing is probably not the most efficient way of going about this. Does anyone know a way around this, or, conversely, what else the problem might be?

Thanks for the help.