Data Communications Networking Devices - 4th Ed.pdf

340 ________________________________________________________ LOCAL AREA NETWORKSFigure 3.36 Destination address sub®elds I/G: individual or group bit address identi®er;U/L: universally or locally administered bit identi®er; FAI: functional address indicator). Thereserved ®eld contains the manufacturer's identi®cation in 22 bits represented by 6 hexdigitsequipment, with the manufacturer encoding a unique address into each adaptercard. Locally administered addressing permits users to temporarily override universallyadministered addressing and can be used to obtain addressing ¯exibility.Functional address indicatorThe functional address indicator sub®eld in the destination address identi®es thefunction associated with the destination address, such as a bridge, active monitor,or con®guration report server.The functional address indicator indicates a functional address when set to 0and the I/G bit position is set to 1, the latter indicating a group address. Thiscondition can only occur when the U/L bit position is also set to 1 and results inthe ability to generate locally administered group addresses that are called functionaladdresses.Source addressThe source address ®eld always represents an individual address which speci®esthe adapter card responsible for the transmission. The source address ®eld consistsof three major sub®elds as illustrated in Figure 3.37. When locally administeredaddressing occurs, only 24 bits in the address ®eld are used since the 22manufacturer identi®cation bit positions are not used.The routing information bit identi®er identi®es the fact that routing informationis contained in an optional routing information ®eld. This bit is set when a frame isrouted across a bridge using IBM's source routing technique which is described indetail in Chapter 5.Figure 3.37 Source address ®eld RI: routing information bit identi®er; U/L: universally orlocally administered bit identi®er). The 46 address bits consist of 22 manufactureridenti®cation bits and 24 universally administered bits when the U/L bit is set to 0. If set to 1,a 31-bit locally administered address is used with the manufacturer's identi®cation bits setto 0