Although an IPID from a logical perspective semantically determines the server, object and interface to which a particular call should be directed, it does not by itself indicate the binding information necessary to actually carry out an invocation.
The protocol represents this "how-to" communication information in a 64-bit value called an object exporter identifier, otherwise known as an OXID. Conceptually, an OXID can be thought of as an implementation scope for an object, which may be a whole machine, a given process, a thread within that process, or other more esoteric implementation scope, but the exact definition of such scopes has no bearing on the protocol itself. Data structures in each Object Exporter keep track of the IPIDs exported and imported by that Object Exporter.
A given machine at any moment may support several OXIDs; however there is always a unique OXID Resolver service per machine which coordinates the management of all the OXIDs on the machine. The OXID Resolver typically (but not necessarily) resides at well-known ports (or endpoints, depending on your terminology -- one per protocol, of course) on the machine. It supports a DCE RPC interface known as IOXIDResolver, described in Section 5.2.
An OXID is used to determine the RPC string bindings that allow calls to reach their target IPID. Before making a call, the calling process must translate an OXID into a set of bindings that the underlying RPC implementation understands. It accomplishes this by maintaining a cache of these mappings. When the destination application receives an object Reference it checks to see if it recognizes the OXID. If it does not, then it asks the OXID Resolver which scopes the OXID specified in the object reference for the translation, and saves the resulting set of string bindings in a local table that maps OXIDs to string bindings.
Associated with each OXID (ie each Object Exporter) is COM object termed an "OXID object." OXID objects implement (at least) the IRemUnknown interface, a COM interface through which remote management of reference counts and requests for interfaces are returned.