IP mobility is software and hardware designed to provide network access as a user travels from network to network. Cisco calls this the ubiquitous connection.

Cellular telephone networks already accomplish this:

You can begin a cellular call in Los Angeles, CA and (in theory) you should be able to drive to New York city without dropping the call (that is, among other things, as long as there are towers along the interstate highways). Granted, it usually doesn't work exactly like this, but it typically works well enough in a metropolitan area.

During that time, the call is moving from cell tower to cell tower, and the network protocols behind the scenes make your network connectivity appear seamless to the end-user.

Mobile IP uses the same concept but applies it to IP network devices on a LAN. The most obvious use for this is in a campus-area network. For example, if you have a wireless PDA, you want to be able to walk from your building to another building, say 100 yards away, without losing network connectivity-or having to reconnect to the wireless network. The two buildings are on separate IP networks, and each has the 10.1.1.0/24 network.

Traditionally, your IP address may need to change when you move from building to building (i.e., network to network). You could use Dynamic Host Configuration Protocol (DHCP), but you would experience a network interruption when receiving a new address and waiting for whatever application you're using to reconnect.

How can you make this process seamless without any such interruptions? Mobile IP has the answer.

Mobile IP is an Internet Engineering Task Force (IETF) standard, outlined in RFC 2002, and a number of other IETF standards also address Mobile IP, including RFC 2003, RFC 2004, RFC 2005, and RFC 2006.

In theory, with a standard protocol there should be interoperability between Mobile IP devices (no matter the brand), assuming they support the standard.

Cisco Mobile IP is a suite of software that runs on different devices, including Mobile IP that runs on IOS routers and the Cisco Mobile Client for Windows.

One Mobile IP feature that you should be familiar with is Local-Area Mobility (LAM). (A Cisco name and password are necessary to access this resource.) This features allows a device to roam from a local IP subnet to another local IP subnet, all while keeping the same IP address. This means you could walk from network to network between buildings on a large campus while using your device.

To determine whether your router supports Mobile IP, go to the Global Configuration Mode prompt of your router, and enter the following:

ip mobile ?

Here's an example:

Cell-Router(config)#ip mobile ?
foreign-agent Foreign Agent services
home-agent Home Agent services
host Grouping of one or more mobile hosts
secure Security association
tunnel Mobile IP tunnel settings
virtual-network IP address of virtual network containing mobile hosts

Cell-Router(config)#ip mobile

You can accomplish most Mobile IP configuration using the ip mobile command. You also use this command to enable Mobile IP routing. Here's an example:

Cell-Router(config)# router mobile

For more information on how Mobile IP works and to learn how to configure it, check out in Cisco's Mobile IP IOS Configuration Guide.

As you might imagine, configuring Mobile IP and preventing users from experiencing any interruptions when roaming from LAN to WLAN to DSL to coffee shop can still be somewhat problematic.

In practice, it's better to start out with a small implementation and work up in complexity and size of the network. Configure Mobile IP on Cisco routers so your users can "roam" from one wireless LAN subnet to another. This is the best use for Mobile IP. This way, users can use PDAs when walking from floor to floor or building to building-in other words, subnet to subnet.

The Cisco IOS mobility software allows you to roam across subnets on a LAN or wireless network, and it works well. But keep in mind that Mobile IP is a relatively new concept for most organizations, and many companies haven't implemented it yet. The concept of being able to seamlessly roam from network to network is a great idea. 

I should add that mobility is changing in IPv6, and encourage you to examine the changes there.

We hope this helps.

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