Yep, that's what I meant by "accessible" too. But with Hamachi, there's a central service for client authentication. With freelan, peers authenticate directly via signed crts.
I looked into this heavily over the past few days. The punchline is this needs support for NAT traversal and some type of out-of-band way for clients to find each other.
NAT traversal is an implementation thing, and I favor Jabber as the out-of-band these days since everyone can get at least a GTalk account.
Though we now have libjingle, which basically merges both of these things and would probably elegantly solve the problem. But p2p vpn's aren't much use if you have to control the NAT router you're attached to.
Doesn't seem to be a problem with connecting with Pidgin, and I had it working with Wippien (sadly Wippien doesn't really work great with Win 7 or Linux).
Google has stopped federating with other Jabber services. They have replaced the GTalk Android app with the Google Hangouts app. They have started pushing people (forcibly) towards Google Hangouts instead of Google Talk. How long before XMPP support is dropped? I don't think we can claim that Google will keep it around for certain.
NAT traversal is an implementation thing, and I favor Jabber as the out-of-band these days since everyone can get at least a GTalk account.
Though we now have libjingle, which basically merges both of these things and would probably elegantly solve the problem. But p2p vpn's aren't much use if you have to control the NAT router you're attached to.