Layer 3 The Network Layer

The network layer determines the best path to a destination. Device addressing, packet fragmentation, and routing all occur at the network layer. Information at this layer is processed in what are commonly known as packets. Examples of network layer protocols include the following:

■ Internetwork Packet Exchange (IPX)

Routing protocols, such as Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP), provide the information required to determine the topology of the internetwork and the best path to a remote destination. A routed protocol is one that is transported by a routing protocol (such as Routing Information Protocol [RIP]). For example, IP is a routed protocol that can be advertised by a number of routing algorithms, such as RIP, OSPF, and BGP. The Layer-3 field format of protocol type defines to the higher layers what protocol is being carried inside the IP packet. For example, OSPF has an IP protocol number of 89; EIGRP has an IP protocol number of 88. Technically, OSPF and EIGRP are not Layer-3 mechanisms.

NOTE Layer 3 protocols, such as IP, are commonly referred to as connectionless protocols, whereas Layer 4 protocols, such as TCP, are commonly referred to as connection-oriented protocols.

A connection-oriented protocol, such as TCP, ensures delivery of all information, whereas a connectionless protocol, such as IP, packages the data but sends it without guaranteeing delivery. Connection-oriented protocols exchange control information (also called Handshake) before transmitting data. A telephone call can be considered a connection-oriented service because the call is established before conversation can take place, much the same way that TCP sets up a data connection before data is sent. FTP is another example of a connection-oriented protocol.

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