Intranet Access through Net

One of my client is having two LANs / Intranet on seperate places in one city itself. Now They have two live IP from both side. Now suppose LAN 1 have PC A B C and LAN 2 have D E F. Here if A wants to see or share files with D E F of LAN 2. But how it can be done ? Because LAN 1 and 2 are connected through internet only by live IP. While ABCDEF are having DEAD IP for LAN.

Any suggestion with 3rd party utilities and without 3rd party utilities ?