In order to invoke the remote object instance, the client first obtains its object reference. (There are many ways to do this, but we won't detail any of them here. Easy ways include the Naming Service and the Trader Service.) To make the remote invocation, the client uses the same code that it used in the local invocation we just described, substituting the object reference for the remote instance. When the ORB examines the object reference and discovers that the target object is remote, it routes the invocation out over the network to the remote object's ORB. (Again we point out: for load balanced servers, this is an oversimplification.)
How does this work? OMG has standardized this process at two key levels: First, the client knows the type of object it's invoking (that it's a shopping cart object, for instance), and the client stub and object skeleton are generated from the same IDL. This means that the client knows exactly which operations it may invoke, what the input parameters are, and where they have to go in the invocation; when the invocation reaches the target, everything is there and in the right place. We've already seen how OMG IDL accomplishes this. Second, the client's ORB and object's ORB must agree on a common protocol - that is, a representation to specify the target object, operation, all parameters (input and output) of every type that they may use, and how all of this is represented over the wire. OMG has defined this also - it's the standard protocol IIOP. (ORBs may use other protocols besides IIOP, and many do for various reasons. But virtually all speak the standard protocol IIOP for reasons of interoperability, and because it's required by OMG for compliance.)
Although the ORB can tell from the object reference that the target object is remote, the client can not. (The user may know that this also, because of other knowledge - for instance, that all accounting objects run on the mainframe at the main office in Tulsa.) There is nothing in the object reference token that the client holds and uses at invocation time that identifies the location of the target object. This ensures location transparency - the CORBA principle that simplifies the design of distributed object computing applications.   _{Your Comment}