Alright, so I'm currently behind a network router (not under my control). The router performs NAT and somehow prevents a computer from scanning other nodes. At least, you're unable, in this instance, to locate an iTunes library. You can, however, communicate with a node's open ports if the local IP address is known, as well as the port. I haven't actually tried port scanning a specific IP using nmap or another tool yet.

So I've tried one solution to remove the contribution of the router entirely (to verify that it works without the influence of the routers). I set up an access point using my iPhone and tethered my computer (with the library) to it. From here, I was able to pair my library and the iPhone Remote application. Control of the library was normal as well.

This solution is not ideal, however, because I am actively using bandwidth with my computer and cannot afford to be tethered to my 3G connection.

A viable solution for me is to use a common VPN connection, which I have set up on a Ubuntu (Intrepid) server that is remote. Both my computer and iPhone are able to access the VPN via PPTP. The server is setup with PPTPD as the VPN-server; I'm using IPTables to perform IP masquerading and forwarding traffic. I however, still cannot connect the library to the phone. I can however, see both devices on the VPN subnet (192.168.0.0/24). SSH'ing and such works fine.

What settings on the VPN server must I change to get this to work? Also, how can I assign static IP addresses to various PPTP clients based on MAC addresses?