Mobile IP (or MIP) is an Internet Engineering Task Force (IETF) standard communications protocol that is designed to allow mobile device users to move from one network to another while maintaining a permanent IP address.
Each mobile node is identified by its home address disregarding its current location in the Internet.
In many applications (e.g., VPN, VoIP), sudden changes in network connectivity and IP address can cause problems.
Examples of use are in roaming between overlapping wireless systems, e.g., IP over DVB, WLAN, WiMAX and BWA.
However, it is often used in 3G systems to allow seamless IP mobility between different packet data serving node (PDSN) domains.
Instead of forwarding these packets to a destination that is physically in the same network as the home agent, the home agent redirects these packets towards the remote address through an IP tunnel by encapsulating the datagram with a new IP header using the care of address of the mobile node.
This is needed in networks whose gateway routers check that the source IP address of the mobile host belongs to their subnet or discard the packet otherwise.