The Network layer reference article from the English Wikipedia on 24-Jul-2004
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Network layer

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OSI Model
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer

The network layer is level three of the seven level OSI model. It responds to service requests from the transport layer and issues service requests to the data link layer.

The network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.

The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions.

The network layer deals with transmitting information all the way from its source to its destination - and transmitting things from anywhere you like, to anywhere you like. If you can't contact a place at the network layer, then you can't contact that place at all. However, it does it in a very basic way, without error detection or flow control, or anything else. Here are some things that the network layer needs to address:

Internet protocol suite
Application layer FTP SMTP HTTP IRC ...
Transport layer TCP UDP SCTP ICMP ...
Network layer IP IPv6 ARP DHCP ...
Data link layer Ethernet Token ring FDDI 802.11 WiFi ...

In the traditional postage system (commonly referred to as snail mail) this role is provided by the postman (to some extent).


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