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CORBA specifies what an ORB should do, not how the ORB vendor should implement it; hence ORB vendors are free to choose how their ORBs are implemented. Under the initial CORBA specification 1.1, there was no standard for ORBs from different vendors to interoperate. This meant that for clients and server objects to interoperate, they had to be using the same ORB, or ORBs from the same vendor. OMG addressed this problem in the CORBA 2.0 specification by defining a standard for how ORBs from different vendors should communicate. The umbrella protocol is called the General Inter-ORB Protocol (GIOP), and the most important specific flavor is the Internet Inter-ORB Protocol (IIOP), which applies to TCP/IP networks. IIOP lets a client make a request to one ORB and have the request relayed through a different ORB to the server object. IIOP leaves you free to choose different ORBs for different parts of your application, and perhaps more importantly, it lets anything that implements IIOP act as a CORBA server or client.
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