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Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model
      9  OSI Reference Model Layers

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Data Link Layer (Layer 2)
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Transport Layer (Layer 4)
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Network Layer (Layer 3)
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Network Layer Connection-Oriented and Connectionless Services

Network layer protocols may offer either connection-oriented or connectionless services for delivering packets across the network. Connectionless ones are by far more common at the network layer. In many protocol suites, the network layer protocol is connectionless, and connection-oriented services are provided by the transport layer. For example, in TCP/IP, the Internet Protocol (IP) is connectionless, while the layer four Transmission Control Protocol (TCP) is connection-oriented.

The most common network layer protocol is of course the Internet Protocol (IP), which is why I have already mentioned it a couple of times. IP is the backbone of the Internet, and the foundation of the entire TCP/IP protocol suite. There are also several protocols directly related to IP that work with it at the network layer, such as IPsec, IP NAT and Mobile IP. ICMP is the main error-handling and control protocol that is used along with IP. Another notable network layer protocol outside the TCP/IP world is the Novell IPX protocol.

Key Concept: The OSI Reference Model’s third layer is called the network layer. This is one of the most important layers in the model; it is responsible for the tasks that link together individual networks into internetworks. Network layer functions include internetwork-level addressing, routing, datagram encapsulation, fragmentation and reassembly, and certain types of error handling and diagnostics. The network layer and transport layer are closely related to each other.


The network interconnection devices that operate at the network layer are usually called routers, which at this point should hopefully come as no surprise to you. They are responsible for the routing functions I have mentioned, by taking packets received as they are sent along each “hop” of a route and sending them on the next leg of their trip. They communicate with each other using routing protocols, to determine the best routes for sending traffic efficiently. So-called “brouters” also reside at least in part at the network layer, as do the rather obviously named “layer three switches”. J


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Data Link Layer (Layer 2)
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Transport Layer (Layer 4)
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