throughout this description , the embodiments and examples shown should be considered as exemplars , rather than limitations on the apparatus and methods of the present invention . referring now to fig1 , there is shown a block diagram of a switching environment 100 . the switching environment includes a switch 110 and a number of end points 120 a , 120 b , 120 c , 120 d . the switching environment 100 may be a point - to - point communications network . the term “ switch ” as used herein means a system element that logically connects two or more ports to allow data units to be routed from one port to another , and the switch 110 is a switch . the switch routes data units using memory - mapped i / o or i / o - mapped i / o ( both , collectively , “ mapped i / o ”). the switch 110 further includes a buffer 115 and logic 117 . the switch 110 includes a number of ports 112 a , 112 b , 112 c , 112 d , which are physical interfaces between the buffer 115 and logic 117 and the end points 120 . by data unit , it is meant a frame , cell , datagram , packet or other unit of information . in some embodiments , such as pci , a data unit is unencapsulated . data units may be stored in the buffer 115 . by buffer , it is meant a dedicated or shared memory , a group or pipeline of registers , and / or other storage device or group of storage devices which can store data temporarily . the buffer 115 may operate at a speed commensurate with the communication speed of the switching environment 100 . for example , it may be desirable to provide a dedicated memory for individual portions ( as described below ) and pipelined registers for multicast portions ( as described below ). the logic 117 includes software and / or hardware for providing functionality and features described herein . the logic 117 may include one or more of : logic arrays , memories , analog circuits , digital circuits , software , firmware , and processors such as microprocessors , field programmable gate arrays ( fpgas ), application specific integrated circuits ( asics ), programmable logic devices ( plds ) and programmable logic arrays ( plas ). the hardware and firmware components of the logic 117 may include various specialized units , circuits , software and interfaces for providing the functionality and features described herein . the invention may be embodied in whole or in part in software which operates in the switch 110 and may be in the form of firmware , an application program , an applet ( e . g ., a java applet ), a browser plug - in , a com object , a dynamic linked library ( dll ), a script , one or more subroutines , or an operating system component or service . the hardware and software of the invention and its functions may be distributed such that some components are performed by the switch 110 and others by other devices . the end points 120 a , 120 b , 120 c , 120 d are logical devices which connect to and communicate with the switch 110 respectively through the ports 112 . at least some of the end points may share an address domain , such as a memory address domain or an i / o address domain . the term “ address domain ” means the total range of addressable locations . if the shared address domain is a memory address domain , then data units are transmitted via memory mapped i / o to a destination address into the shared memory address domain . the end points 120 may be connected to the ports 112 by electrical contacts , wirelessly , optically or otherwise . referring now to fig2 , there is shown a diagram of two address domains 200 , 250 . one address domain 200 is shared by end points 120 a , 120 b , 120 d , and the other address domain 250 is not shared and used only by end point 120 d . this is just an example ; there may be more than two address domains , and more than one address domain may be shared . the address domains 200 , 250 are contiguous ranges . each address domains is defined by a master end point . address portions associated with the individual end points 120 may be non - contiguous and the term “ portions ” is meant to refer to contiguous and non - contiguous spaces . the master end point for a given address domain allocates address portions to the other end points which share that address domain . the end points communicate their address space needs to the master device , and the master device allocates address space accordingly . data units may be written into or communicated into an address portion . in a switch conforming to the pci express standard , it is expected that the address portions in a 32 - bit shared memory address domain or shared i / o address domain will be at least as large as the largest expected transaction , and comparable to those shown in fig2 . within the shared address domain 200 , separate address portions 210 a , 210 b , 210 c may be associated with the corresponding end points 120 a , 120 b , 120 c . the address domain 200 may be allocated so as to provide the corresponding end points 120 a , 120 b , 120 c with unique address portions . the address portions may be unique within the shared address domain 200 with respect to one another . within the non - shared address domain 250 , there may be a portion 250 d associated with the end point 120 d . the non - shared address domain 250 is considered isolated from the shared address domain 210 . other non - shared address domains could be included , and they would also be considered isolated from the shared address domain , and from each other . by “ isolated ” it is meant that the address domains are separated such that interaction does not directly take place between them , and therefore uniquely addressable addresses are provided . the address portions 210 may have various characteristics . the address portions 210 may have respective sizes . the sizes may be fixed or variable . the address portions 210 may be defined by a base address , as well as by a size or end address . the address portions 210 may come to be associated with the end points 120 through an arbitrage process , through centralized assignment ( e . g ., by a host or the switch 110 ), otherwise or through a combination of these . the address portion 210 for a given end point 120 need not be contiguous . to avoid errors , it may be desirable if the address portions 210 within the same address domain do not overlap . data units may be directed to one or more of the end points 120 by addressing . that is , a destination address is associated with and may be included in the data units . the destination address determines which end point 120 should receive a given data unit . thus , data units addressed to the individual portion for a given end point 120 should be received only by that end point 120 . depending on the embodiment , the destination address may be the same as the base address or may be within the address portion . the end points 120 may be associated with respective ports 112 . through this association , a given end point 120 may send data units to and receive data units from its associated port 112 . this association may be on a one - to - one basis . because of these relationships , the ports 112 also have associations with the address portions 210 of the end points 120 . thus , the ports 112 may be said to have address portions 210 within the address domains 200 , 250 . ports within a shared addressed domain are considered “ transparent ”, and those not within a shared address domain are considered “ non - transparent ”. data units from one transparent port to another may be transferred directly . however , data units between a transparent port and a non - transparent port require address translation to accommodate the differences in their respective address domains . transparent ports are logical interfaces within a single addressing domain . non - transparent ports allow interaction between completely separate addressing domains , but addresses from one domain must be converted from one domain to the other . the status of a port — transparent or non - transparent — may be fixed or configurable . the logic 117 may allow designation on a port - by - port of transparency or non - transparency , including the address domain for a given port . the switch 110 may be responsive to requests or instructions from the devices 120 to indicate such things as which address domain the devices will be in , and the address portion associated with a given device . referring now to fig3 there is shown a flow chart of a process for switching data units . the process employs a switch having transparent and non - transparent ports , such as the switches described above . in the switch , the transparent ports are associated with a shared address domain , and the non - transparent ports are associated with non - shared address domains . domain maps for each address domain may be communicated to the switch . there may be provided a master end point , such as a processor , which is responsible for allocating address portions within its address domain . end points may communicate their address space needs to the master device , and the master device may allocate address space accordingly . the master device may query end points for their address space needs . these allocations , and other allocations and designations , define the address map which the master end point communicates to the switch . the switch may receive a single communication of an address map from a master end point . the switch may receive partial or revised address maps from time to time . in a first step 305 , the switch receives a data unit . the switch then stores the data unit in a buffer ( step 310 ). next , the switch determines the destination address of the data unit ( step 315 ). next , the switch determines whether the destination address is associated with a transparent or non - transparent port ( step 325 ). if the address is associated with a non - transparent port , then the switch translates the address ( step 330 ). many different schemes of memory and i / o address translation for mapping from one address domain into another may be used . these schemes include direct memory translation both with and without offsets , and indirect memory translation through lookup registers or tables . furthermore , addresses may be translated using schemes other than address map translation , such as mailbox mechanisms and doorbell registers . whether or not translated , the switch forwards the data unit to the port for the designated destination address ( step 395 ). in this way , data units are transferred between the transparent ports , between the transparent and non - transparent ports , and between the non - transparent ports . in effect , non - transparent ports allow data transfers from one address domain to another . in one embodiment , the switch is a pci express switch in which one or more of the interfaces ( i . e ., ports ) are optionally non - transparent . a device connected to a non - transparent port of the switch is isolated from the address domain of the other ports on the switch . two or more processors with their own address maps could all communicate with each other through this type of pci express switch . with regard to fig3 , additional and fewer steps may be taken , and the steps as shown may be combined or further refined to achieve the methods described herein . non - transparent operation allows a local subsystem to maintain a full address range completely separate from the main system . in addition , the presence of the local bus is obfuscated from the main system by presenting the non - transparent bridge as an endpoint . bus enumeration and discovery software remains unaware of the presence of the secondary local bus , allowing for a higher level of abstraction at the system level . to provide both transparent and non - transparent ports , a transparent bridge may be associated with a different type of configuration space than a non - transparent bridge . upon discovering the transparent type configuration space , bus enumeration and discovery software may read “ through ” the bridge device in an attempt to identify additional downstream devices . a non - transparent bridge , on the other hand , masks the presence of the secondary local bus by identifying itself as an endpoint . the endpoint association is made via a non - transparent type configuration header . upon discovering a non - transparent type configuration space , bus enumeration and discovery software is satisfied and does not attempt to read through the non - transparent bridge . a pci express switch ( including a bridge as described herein ) in non - transparent mode allows completely independent and unrestricted address ranges to exist on both sides of the bridge . transactions passing through the bridge will have their addresses remapped to a correlating destination address . a local host processor may be responsible for maintaining address translation and configuration registers . when address configuration is complete , a primary side lockout bit may “ wake ” the primary pci express endpoint interface , allowing it to respond to bus discovery queries . addresses may be remapped in a three - stage process . first , the size and configuration of each address base address range is set by the local host . this allows software agents on the main host to allocate and distribute the main memory map . next , the local host programs the translation offset . finally , software agents on each side of the bridge perform standard pci address mapping techniques to map the respective address regions into system memory space . upon receiving a transaction that falls within a given base address range , a translation function overwrites the upper bits of the address with the translated base offset , or some other address translation technique . thus , separate address ranges are maintained on both sides of the non - transparent bridge . a downstream range may be associated with requests moving from the pci express to the pci interface . an upstream range is associated with request moving from the local pci bus to the pci express interface . although exemplary embodiments of the present invention have been shown and described , it will be apparent to those having ordinary skill in the art that a number of changes , modifications , or alterations to the invention as described herein may be made , none of which depart from the spirit of the present invention . all such changes , modifications and alterations should therefore be seen as within the scope of the present invention .