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.The first 6 hexadecimal digits, which are administered bythe IEEE, identify the manufacturer or vendor and thus comprise the Organizational UniqueIdentifier (OUI).The last 6 hexadecimal digits comprise the interface serial number, or anothervalue administered by the specific vendor.MAC addresses sometimes are called burned-inaddresses (BIAs) because they are burned into read-only memory (ROM) and are copied intorandom-access memory (RAM) when the interface card initializes.Network Layer AddressesA network layer address identifies an entity at the OSI network layer.Network addresses usuallyexist within a hierarchical address space and sometimes are called virtual or logical addresses.Network layer addresses have two parts: the network that the device is on and the device (orhost) number of that device on that network.Devices on the same logical network must haveaddresses with the same network part; however they will have unique device parts.This is analogous to the postal network addresses: one part indicates the street, city,province/state, and so on, whereas the other part identifies the building number on that street.For example, a building at 27 Main Street is on the same network as a building at 35 MainStreet.The network portion of their addresses, Main Street, is identical, whereas the device portions are unique. CH01.book Page 517 Friday, January 7, 2000 5:35 PMNetwork Devices 517Network DevicesThe main devices used in networking are as follows:" Hubs" Bridges and switches" RoutersHubsA hub is used to connect devices so that they are on one LAN, as shown in Figure C-2.Thecables normally used for Ethernet have RJ-45 connectors.Because only two devices can beconnected with these cables, we need a hub if we want to interconnect more than two deviceson one LAN.Figure C-2 A Hub Connects Devices So that They Are on One LANHubPC Sun Host MacA hub is not a smart device.A hub sends all the data from a device on one port to all the otherports.When devices are connected via a hub, the devices all hear everything that the otherdevices send, whether it was meant for them or not.This is analogous to being in a room withlots of people if you speak, everyone will hear you.If more than one person speaks at a time,there will just be noise.Rules must be put in place if real conversations are to happen; innetworking, these rules are the protocols.Bridges and SwitchesTo improve performance, LANs are usually divided into smaller multiple LANs.These LANsare then interconnected by a LAN switch or by a bridge, as shown in Figure C-3. CH01.book Page 518 Friday, January 7, 2000 5:35 PM518 Appendix C: Internetworking Technology ReviewFigure C-3 LANs Are Split into Many Smaller LANs, Using Switches or Bridges to Improve PerformanceSwitches and bridges have some smarts. When devices are connected via a switch or a bridge,a device will only hear the following:" Everything that the other devices on its port send" Any information from devices on other ports that was meant for everyone (a broadcast)" Any information from devices on other ports that was meant for devices on its portA device connected to a switch or bridge will not hear any of the information meant just fordevices on other ports of the switch.Upper-layer protocol transparency is a primary advantage of both bridging and switching.Because both device types operate at the data link layer, they are not required to examine upper-layer information.This means that they can rapidly forward traffic representing any networklayer protocol.It is not uncommon for a bridge to move AppleTalk, DECnet, TCP/IP, XNS, andother traffic between two or more networks.By dividing large networks into self-contained units, bridges and switches provide severaladvantages.Because only a certain percentage of traffic is forwarded, a bridge or switchdiminishes the traffic experienced by devices on all connected segments.Bridges and switchesextend the effective length of a LAN, permitting the attachment of distant stations that were notpreviously permitted.Bridging ProtocolsSwitches and bridges may communicate with each other by using a bridging protocol.Severaltypes of bridging protocols are supported by Cisco routers (acting as bridges) and switches,including the following:" Transparent bridging Found primarily in Ethernet environments." Source-route bridging (SRB) Found primarily in Token Ring environments." Translational bridging Translates from Ethernet bridging to Token Ring bridging. CH01.book Page 519 Friday, January 7, 2000 5:35 PMNetwork Devices 519" Encapsulating bridging Allows packets to cross a bridged backbone network." Source-route transparent (SRT) bridging Allows a bridge to function as botha source-routing and transparent bridge." Source-route translational (SR/TLB) bridging Allows a bridge to functionas both a source-routing and transparent bridge, and to bridge between the two.NOTE In SRB terminology, Layer 2 frames are also known as packets.Transparent bridges send Bridge Protocol Data Unit (BPDU) frames to each other to build andmaintain a spanning tree, as specified in IEEE 802.1d.The Spanning-Tree Algorithm states thatthere is one and only one active path between two stations.If a physical loop exists in thenetwork (for redundancy reasons), the Spanning-Tree Algorithm handles this loop by disablingbridge ports.This prevents broadcast storms in networks with redundancy, which occurs whenbroadcasts continuously circle the network.Bridges Versus SwitchesAlthough bridges and switches share most relevant attributes, several distinctions differentiatethese technologies.Switches are significantly faster because they switch in hardware, whereasbridges switch in software
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