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. Well 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 LANs, in a WAN environment.
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, youll 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, its 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.
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