From the inside? Outside don't you mean?
It certainly is possible, I've seen it done. I'm fairly sure you would use a VPN, but how to set one up, I've no idea.
I've never set one up before, so I can't really make sense of this, but
http://www.tldp.org/HOWTO/VPN-HOWTO/ looks like a good starting place.
From that document:
2.2. But really, what IS a VPN?
There are several answers to that question. It really depends on your network layout. The most common configuration is to have a single main internal network with remote nodes using VPN to gain full access to the central net. The remote nodes are commonly remote offices or employees working from home. You can also link two small (or large) networks to form an even larger single network.
