APN vs VPN
A VPN is virtually private because the data traveling between offices shares the same paths as other public data, though it is encrypted and ‘tunneled’ between multiple locations to give the appearance and functionality of physically separate wires. The common approach is to purchase a variety of Internet connections, like DSL, Frame Relay, or T1's, from any public resource, carriers and/or independent ISP's. Then without those vendor's specific cooperation, additional hardware or software is employed to 'tunnel' and hide private data between each office. The downside is that it can be expensive. A reasonable Cisco VPN device can exceed $1000 or more and it slows down the transfer while the packets are encrypted, tunneled, and decrypted. Its also difficult to debug and support when there are problems because multiple vendors are involved who don't have any responsibility for connectivity access except to say that Internet access appears to be available.