[TriLUG] Tunnel two networks together?

[bp]

  Will some sort of  VPN between Linux boxes on the internet bridge 
their internal class C networks?  And to make it interesting, only one 
is visible on the internet so box B would have to initiate any connection.

The *need*  is to allow Samba/Windows shares to be accessable in either 
direction between the two networks.  

Host A is accessable via the internet but can not reach host B because 
of service provider firewalls or router rules.  Host B can contact host 
A on any port.  I expect that their is some sort of swan deployment that 
would allow Host B to open a tunnel to Host A's network.

What FAQ do I read or what software do I need to configure?

Would host A & B need to be on the same class C subnet?  Or would you 
just route one C subnet across the tunnel to another C subnet?

Thanks,
bp