token-passing  access  method.  However,  we  cannot connect a CSMA/CD segment to a token-passing segment. Bridges Bridges handle the first two layers of the OSI model—the  physical  layer  and  the  data  link  layer.  Like repeaters,  bridges  connect  physically-isolated  networks to forma single logical network; however, a bridge has a little   more   intelligence   and   can   provide   some translation between dissimilar protocols. For example, our token-passing segment wants to communicate with our CSMA/CD segment. The bridge will “repackage” the  message  from  the  token-passing  segment  into  a format that the CSMA/CD segment will understand. Then, the bridge will act as a workstation on the CSMA/CD segment and contend for access. The same thing happens in reverse. A message is sent from the CSMA/CD segment to the token-passing segment. The bridge  then  “repackages”  the  message  into  a  format  the token-passing segment is expecting and waits for the token, just like any other workstation. An important point to remember is that a bridge will pass on any message it receives. Because the bridge is not smart enough to know that unlike LANs do not understand each other, it will go ahead and send the message. Because  the  two  LANs  speak  a  different  “language,” the message will be ignored. Routers Routers  only  connect  networks  running  similar access methods. They work at the third layer of the OSI model—the network layer. Like bridges and repeaters, routers can connect networks over different wiring media  and  topologies.  However,  unlike  bridges,  routers can intelligently determine the most efficient path to any  destination,  based  on  predetermined  delimiters. Routers are often a better choice for interconnecting remote  installations  and  congested  networks  requiring  a single protocol. Let’s look at this more closely. Let’s say we have a LAN made up of three token- passing segments, and each segment is connected via a bridge. For a message to go from LAN A to LAN C, it would have to travel through LAN A and LAN B before it reaches its final destination, which is LAN C. See figure 2-2, frame A. On a LAN that has large amounts of message traffic, we can see how a bridge may slow down the system. On the other hand, if the segments are separated by routers, the router on LAN A would look at the destination of the message and determine the direct route  to  LAN  C  that  would  be  shortest  route,  as shown in figure 2-2, frame B. Brouters A brouter can work in either the second and third layers of the OSI model—the data link layer or the network layer. A brouter is a combination of a bridge and router combined. If it can’t route a packet, it acts as a bridge. Brouters are particularly useful if you have two or more different networks. Working as a bridge, a brouter is protocol independent and can be used to filter local are a network traffic. Working as a router, a brouter is capable of routing packets across networks. Gateways Gateways   work   at   OSI   model   layer   7—the application  layer.  A  gateway  functions  to  reconcile differences   between   two   dissimilar   networks. Messages  are  not  only  repackaged  for  transmission between different networks (CSMA/CD to token- passing), but the contents of the messages are converted into a format the destination can use and understand. Now our unlike LANs can talk to each other. Gateways can   also   provide   links   between   microcomputer networks and mainframes. A gateway is generally a dedicated computer with an interface card and at least some type of software for both  of  the  environments  being  connected.  The gateway then runs special software that provides the necessary  conversion  and  translation  services  which,  in turn, allow the two environments to communicate. Figure  2-2.—Interconnecting  LANS  using  (A)  bridges  and  (B) routers. 2-2