Networking and Internetworking Devices

1. When two or more separate networks are connected for exchanging data or resources, they become an internetwork.

2. Networking and internetworking devices are divided into four categories: repeaters, bridges, routers, and gateways.
   • A repeater is an electronic device that operates on only the physical layer of the OSI model. It regenerates a signal.

   • Bridges operate in both the physical and data link layers of the OSI model. Bridges can divide a large network into smaller segments. Bridges contain logic that allows them to keep the traffic for each segment separate. They have access to the physical addresses of all stations connected to them.
     • Simple bridges link two segments and contain a table that lists the addresses of all the stations included in each of them. These addresses must be entered manually.

     • Multiport bridges can be used to connect more than two LANs.

     • A transparent, or learning bridge builds its table of station addresses on its own as it performs its bridge function.

   • Routers have access to network layer addresses and contain software that enables them to determine which of several possible paths between those addresses is the best for a particular transmission. Routers operate in the physical, data link, and network layers of the OSI model.
     • Least-cost routing is based on efficiency, it selects the cheapest, or shortest available pathways.

     • Nonadaptive routing is when a selected pathway to a destination is the same for all packets.

     • Adaptive routing is when a new route may be selected for each packet in response to changes in condition and topology of the networks.

   • Gateways are protocol converters that potentially operate on all seven layers of the OSI model.

   • Multiprotocol routers route packets belonging to two or more protocols.

   • A brouter (bridge/router) is a single or multiprotocol router that can be either a router or a bridge.

   • A store-and-forward switch stores the frame in the input buffer until the whole packet has arrived. A cut-through switch forwards the packet to the output buffer as soon as the destination address is received.

   • Routing switches are a combination of router and bridge that use the network layer destination address to find the output link to which the packet should be forwarded.

3. Routing algorithms
   • Distance vector routing is where each router periodically shares its knowledge about the entire network with its neighbors.

   • Link state routing is where each router shares its knowledge of its neighborhood with every other router in the internetwork.

   • To calculate its routing table, each router applies the Dijkstra algorithm to its link state database. The Dijkstra algorithm calculates the shortest path between two points on a network using a graph made up of nodes and arcs.