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Traditionally, IP has assumed that a host on the Internet always connects to the same point of attachment. Any person or system that wants to send datagrams to that host addresses the datagrams to an IP address that identifies the subnetwork where the host is normally located. If the host moves, it will not receive those datagrams.

Today, a growing number of Internet users move their systems from place to place. If you normally connect to an ISP (Internet service provider) to establish an Internet connection and receive Internet mail, you'll need to dial long distance into that ISP if you travel to another state or country. The alternative is to dial in via a VPN connection or to have a different IP address at your destination location, but this does not help if people are used to contacting you at another IP address.

Mobile IP, as defined in IETF RFC 2002 (IP Mobility Support, October 1996), provides a mechanism that accommodates mobility on the Internet. It defines how nodes can change their point of attachment to the Internet without changing their IP address.

Mobile IP assumes that a node's IP address remains the same as it is moved from one network location to another. It also allows a user to change from one media type to another (for example, from Ethernet to a wireless LAN). This is not to be confused with "roaming," which allows users to roam among groups of Internet service providers while maintaining a user account with just one of those providers. Roaming is concerned with the movement of users, not hosts or subnets. See "Roaming."