http://searchmobilecomputing.techtarget.com/definition/mobile-VPN
Mobile VPN solutions from such vendors as Columbitech, Ecutel,
IBM, ipUnplugged, Motorola, NetMotion, Nokia, Padcom and Radio IP are designed to adapt transparently to these changes. In a mobile VPN, a VPN server still sits at the edge of your company network, enabling secure tunneled access by authenticated, authorized VPN clients. Mobile VPN tunnels are not tied to physical IP addresses, however. Instead, each tunnel is bound to a logical IP address.
That logical IP address sticks to the mobile device no matter where it may roam. For example, a mobile VPN client can:
Roam from one wireless AP to another at a public Wi-Fi hot spot.
Leave Wi-Fi coverage and start using a 3G connection (e.g., EV-DO).
Leave 3G coverage and start using a slower 2G connection (e.g., 1xRTT).
Return to the office and start using a docked Ethernet LAN connection.
In this example, the mobile VPN client uses four or five different physical IP addresses while retaining one logical IP address. Applications running on the mobile device and inside the corporate network communicate through that one logical IP address, remaining blissfully unaware of the user's motion and associated physical/network transitions.
Hoping someone with some computer tech knowledge can chime in.