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Table Of Contents  CertiGuide to Network+
 9  Chapter 0010:  ISO, OSI, and IEEE Standards
      9  II  Open Systems Interconnect

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Layer 2 - Data Link Layer
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Layer 4 - Transport Layer
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Layer 3 - Network Layer

The Network layer routes packets for addresses that are not on the your LAN. Unlike the 2nd layer, the network layer operates on routable protocols (protocols which can span networks) to deliver data across interconnected networks, typically known as internetworking. We’ll discuss routable protocols in more detail in the chapter on network protocols. For now, be aware that layer 3 manages the transmission of data across multiple connected LAN’s, in a WAN environment.

Protocol Defined

A protocol is an agreed-upon format for transmitting data between two or more devices on a network.

The functions at each OSI layer are implemented by one or more communication protocols designed to handle that function in a way that all devices understand.

Much of Networking involves an understanding of protocols.


The addresses (unique network device identifiers) used by the network layer to manage the transmission of data are generally configurable in software, and easily changeable, as opposed to the hard-wired addresses used by the physical layer. The format of the addresses is determined by the networking standard in use on the LAN.

Changing from one network layer standard to another (for example, from IPX/SPX to TCP/IP) would result in the need to update the addresses used at this level, but not the physical addresses used in layer 2 communications. Once again, you’ll hear more about this when we discuss network protocols.

Some of the “traffic direction” functions involved in getting data from one host to another on a LAN can be handled in either layer 2 or layer 3. Generally, for improved speed, it’s best to handle these functions at layer 2 if possible, since layer 2 processing tends to be simpler, thus making processing at layer 2 generally faster than processing at layer 3.

Figure 27: Layer 3 - Network Layer

 


Layer 3

In layer 3 (network layer) we find network addresses and hardware that routes data among multiple networks (as opposed to among devices on a single LAN).



Previous Topic/Section
Layer 2 - Data Link Layer
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Layer 4 - Transport Layer
Next Topic/Section

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