A traditional Internet protocol (IP) address plan calls for a single subnet (define) for the entire wireless LAN (WLAN). This is simple to implement, and it provide no barriers to effective user roaming. Some companies, however, want to implement multiple IP subnets across a common WLAN in order to make network management easier, facilitate location-based services, and decrease the spread of broadcast packets throughout the network.

For example, a company may want to deliver specific information to users based on their location in a specific building. By designating different subnets throughout the WLAN, the location of the user can be found and content delivered to the user based on where they are. The location of the user in an airport, for instance, could deliver a map of the applicable concourse or terminal, indicating flight information and the location of coffee shops and ticket counters. The system could also deliver advertisements from concessions located within the general area.

With multiple subnets, however, mobile users must be able to seamlessly roam from one subnet to another while traversing a facility. WLAN access points do a great job of supporting roaming at Layer 2 (define). Users automatically associate and reassociate with different access points (define) as they move through a facility. As users roam across subnets, though, there must be a mechanism at Layer 3 to ensure that the user device configured with a specific IP address can continue communications with applications.

Mobile IP (define), offered by some access point vendors, solves this problem by allowing the mobile user to use two IP addresses. One address, the home address, is static. The second address, the "care-of" address, changes at each new point of network attachment and can be thought of as the mobile user's position-specific address.

The home address enables the mobile node to continually receive data relative to its home network, through the use of a network node called the home agent. Whenever the user is not attached to the home network, the home agent receives all the packets sent to the mobile user and arranges to deliver them to the mobile user's current point of attachment, which is its care-of address.

Whenever a Mobile IP user moves, it registers its new care-of address with its home agent. This makes it possible for the home agent to keep up with the whereabouts of the mobile user. The home agent then sends any packets it receives for that user to the applicable care-of address.

In order to implement Mobile IP, you need two major components: a Mobile IP server and Mobile IP client software. The Mobile IP server will fully implement the Mobile IP home agent functionality, providing mobility management for the mobile users. The Mobile IP server can generally also keep track of where, when, and how long users utilize the roaming service. That data can then provide the basis for accounting and billing purposes.

The requirement for client-side software makes Mobile IP impractical for some applications. For example, public networks demand open connectivity for users, which makes it difficult to deploy solutions that require client software. The task of installing software on user devices before enabling roaming is too cumbersome.
Another problem with Mobile IP is that it is somewhat vendor specific. To ensure interoperability among multi-vendor Mobile IP clients and servers, definitely do some up front testing.

Jim Geier is an independent consultant and founder of Wireless-Nets, Ltd (www.wireless-nets.com), a consulting firm assisting municipalities, enterprises, hospitals, airports, and equipment providers with the development and deployment of wireless networks.