8. THE NETWORK LAYER OF THE OSI MODEL

Description of the Network layer

The network layer defines protocols that ensure that the data arrives at the correct destination. The two most important network layer concepts are:

1. Logical Networking Addressing: Just as the data link layer uses MAC addressing. The networking layer uses protocol addressing to define the logical address. This means that each logical network address is protocol dependent; a TCP/IP (Transmission Control Protocol/Internet Protocol) address, for instance, is not the same as an IPX (Internet Packet EXchange) address.
2. Routing: This is the process of moving data between computers and through a network. The data passes through several network segments using devices called routers that can select the data's pathway. When routers are used to connect several smaller routers that network is called an internetwork. Routers use routing tables to decide which path to send the data on. These tables contain the information about which port to send it in order for it to arrive at a particular network segment. A router will only route a packet if it has a specific destination. Information is put into routing tables in one of two ways:

1. Static Routing: The network administrator manually updates the routing table by entering every network segment and their respective port assignments. On a large network this is time-intensive and complicated.
2. Dynamic Routing: Router discovery protocols are used in dynamic routing. This means that routers on a network "talk" to one another to find out which networks they are attached to.

There are two categories of route discovery protocols:

1. Distance Vector: This is the older protocol of the two and is used by the Routing Information Protocol (RIP) for TCP/IP and IPX. In this protocol when a router first comes online and every sixty seconds afterwards it broadcasts the contents of the routing tables. When another router receives this list it adds a hop to each of the routers on the list and then rebroadcasts the list. A hop is one pass through a router. The main problem with this type of discovery protocol is the pull on network resources for the routing table broadcasts.
2. Link State: In this route discovery protocol the routing table is sent by the more efficient multicast method rather then the broadcast method used by distance vector discovery protocols. Furthermore, rather then the entire routing table being sent, link state technology sends updates, and these, only every five minutes.

Network Layer Devices

Three devices operate at the network layer:

1. Routers: As discussed earlier a router is a device that connects multiple networks into internetworks. It makes choices on how to direct data packets across a network so that they arrive at their destination. Routers can connect dissimilar lower layer topologies, such as Ethernet and Token Ring, to one another. Also, with added software, routers can perform firewall and packet filtering functions.
2. Brouters: A brouter combines the functionalities of a bridge and a router. They aren't used as much anymore and its primary function was to take packets that could not be routed and attempted to bridge them. It was mainly used to connect different network topologies.
3. Layer 3 Switches: Called, Layer 3 because they operate at the third (network) layer of the OSI model. It performs basic routing functions between virtual LANs and in some workgroups can be used in place of a router.

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