net use – mapping drives over the net

Greetings,

I’ve noticed that I am able to sometimes map a drive over the internet, for example: ‘net use z: \\124.23.14.12’ and sometimes I am not able to.

For example I have a client company that has a cisco firewall that does NAT’ing that seperates me from the client. All ports are open. I am able to map a drive over the internet.

My question is: How does driving mapping work over the internet? is it RPC over IP? What port(s) does it use? If I have a router that has all ports open , is there anything else that would stop it from working?

Thank you for your time.