How do COM Interface pointers relate to the implementing class?

Say I have a COM Interface pointer

IMyInterface *pInterface = 0x12696340;

I can see from the debugger by following the function call that this must be implemented by CMyImplementer, but that class is at 0x12686e50.

My question is, how does COM relate the interface to the implementing class? There must be some way to convert from the interface to the class pointer - how would one do that?