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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 13: Basic Networking Terminology
      9  OSI Reference Model and Networking Protocols and Technologies
           9  Layer 3 - Network Layer

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Dynamic Addressing and DHCP
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Layer 4 - Transport Layer
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IPX/SPX and AppleTalk

Protocols such as IP, IPX/SPX and AppleTalk provide Layer 3 support and are, therefore, routable. However, there are protocols that do not support Layer 3; these are classed as non-routable protocols. The most common of these non-routable protocols is NetBEUI. NetBEUI is a small, fast, and efficient protocol that is limited to running on one segment.

In order for a protocol to be routable, it must provide the ability to assign a network number, as well as a host number, to each individual device. Some protocols, such as IPX, only require assignation of a network number, because they use a host's MAC address for the physical number. Other protocols, such as IP, require providing a complete address, as well as a subnet mask. The network address is obtained by adding the address with the subnet mask

Routing protocols (do not confuse with routed protocols) determine the paths that routed protocols follow to their destinations. Examples of routing protocols include the Routing Information Protocol (RIP), the Interior Gateway Routing Protocol (IGRP), the Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF).

Routing protocols enable routers that are connected to create a map, internally, of other routers in the network or on the Internet. This allows routing (i.e. selecting the best path, and switching) to occur. Such maps become part of each router's routing table.

Two types of routing protocols are the Exterior Gateway Protocols (EGPs) and the Interior Gateway Protocols (IGPs). Exterior Gateway Protocols route data between autonomous systems. An example of an EGP is BGP (Border Gateway Protocol), the primary exterior routing protocol of the Internet.

Most network services use a connectionless delivery system. They treat each packet separately, and send it on its way through the network. The packets may take different paths to get through the network, but they are reassembled when they arrive at the destination. In a connectionless system, the destination is not contacted before a packet is sent. A good analogy for a connectionless system is a postal system. The recipient is not contacted before a letter is sent from one destination to another. The letter is sent on its way, and the recipient learns of the letter when it arrives.

In connection-oriented systems, a connection is established between the sender and the recipient before any data is transferred. An example of a connection-oriented network is the telephone system. A person places a call, a connection is established, and then communication occurs.


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Dynamic Addressing and DHCP
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Layer 4 - Transport Layer
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