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 being processed at this layer is commonly known as packets. Examples of network layer protocols include the following:

• Open Shortest Path First (OSPF)

• Cisco's EIGRP routing protocol

Routing protocols (OSPF, EIGRP, and BGP, for example) 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 RIP). For example, IP is a routed protocol that can be advertised by a number of routing algorithms, such as RIP, OSPF, and BGP.

NOTE Connection-oriented and connectionless protocols are commonly used terms to describe Layer 3 and 4 (lower layers of the OSI model) protocols, such as IP or TCP. A connection-oriented protocol, such as TCP, ensures delivery of all information, whereas a connectionless protocol, such as IP, only packages the data and 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. IP is an example of connectionless service.

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