Method and system for address routing in a distributed computing system, such as a distributed computing system that uses PCI Express protocol to communicate over an I/O fabric. A destination identifier is provided to identify a physical or virtual host or end point. When a physical or virtual host or end point receives a PCI data packet it compares a list of source identifiers with destination identifiers to determine if a source identifier included in the transaction packet is associated with a destination identifier included in the transaction packet to determine if the transaction packet has a valid association. If the transaction packet has a valid association, it is routed to the target device. The present invention enables each host that attaches to PCI bridges or switches and shares a set of common PCI devices to have its own PCI 64-bit address space and enables the routing of PCI transaction packets between multiple hosts and adapters, through a PCI switched-fabric bus using a destination identifier.