Method, apparatus and system for isolating input/output adapter Direct Memory Access addressing domains in a data processing system. The data processing system includes a plurality of input/output adapters, and access to a memory of the data processing system by the plurality of input/output adapters is controlled by functionality in a host bridge that connects the plurality of input/output adapters to a system bus of the data processing system, thus permitting the use of low cost, industry standard switches and bridges external to the host bridge.