This document defines mobility and multihoming extensions to the Host Identity Protocol (HIP). Specifically, this document defines a general "LOCATOR" parameter for HIP messages that allows for a HIP host to notify peers about alternate addresses at which it may be reached. This document also defines elements of procedure for mobility of a HIP host---the process by which a host dynamically changes the primary locator that it uses to receive packets. While the same LOCATOR parameter can also be used to support end-host multihoming, detailed procedures are left for further study.