Domain trusting

I have a Win2k network at home (native mode) and my friend has a Win2k network at home (mixed mode). We both have xDSL high speed internet access but we’re using different IPS’. We tried to trust each other’s domain but it hasn’t worked. The error message was “not able to contact domain”. I opened port 138 and 750 on my Lynksys router but he still couldn’t contact my domain, and he shutdown his blackice but I couldn’t contact his domain also. Does anyone knows how to do it?