Internetworking protocols such as IP currently do not allow 'mobile hosts' to interoperate easily or conveniently with other hosts on the network. A host's IP address encodes the network number to which the host is connected, which prevents IP datagrams from reaching the host when it moves to a new location and connects to the Internet within a different network. This paper presents a new protocol for allowing mobile hosts to transparently interoperate in the Internet using IP. The protocol is designed to make use of existing facilities of the IP protocol architecture where possible, in order to minimize any changes necessary to existing protocol software. The protocol takes advantage of the standard IP loose source routing option for routing datagrams correctly to mobile hosts, while allowing the hosts to retain their normal 'home' IP address even when connected to a foreign network. The protocol is simple and scales well to large numbers of mobile hosts. It requires fewer changes to existing software and adds less overhead to the network than previous IP mobile host internetworking protocols.