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Network Layer (Layer 3)
(Page 1 of 2)
The third-lowest layer of the OSI
Reference Model is the network layer. If the data link layer
is the one that basically defines the boundaries of what is considered
a network, the network layer is the one that defines how internetworks
(interconnected networks) function. The network layer is the lowest
one in the OSI model that is concerned with actually getting data from
one computer to another even if it is on a remote network; in contrast,
the data link layer only deals with devices that are local to each other.
While all of layers 2 through 6 in
the OSI Reference Model serve to act as fences between the
layers below them and the layers above them, the network layer is particularly
important in this regard. It is at this layer that the transition really
begins from the more abstract functions of the higher layerswhich
don't concern themselves as much with data deliveryinto the specific
tasks required to get data to its destination. The transport
layer, which is related to the network
layer in a number of ways, continues this abstraction transition
as you go up the OSI protocol stack.
Network Layer Functions
Some of the specific jobs normally
performed by the network layer include:
- Logical Addressing: Every device that
communicates over a network has associated with it a logical address,
sometimes called a layer three address. For example, on the Internet,
the Internet Protocol (IP) is the network layer protocol and every machine
has an IP address. Note that addressing is done at the data link layer
as well, but those addresses refer to local physical devices. In contrast,
logical addresses are independent of particular hardware and must be
unique across an entire internetwork.
- Routing: Moving data across a series of
interconnected networks is probably the defining function of the network
layer. It is the job of the devices and software routines that function
at the network layer to handle incoming packets from various sources,
determine their final destination, and then figure out where they need
to be sent to get them where they are supposed to go. I discuss routing
in the OSI model more completely in this topic on the
topic on indirect device connection, and
show how it works by way of an OSI
- Datagram Encapsulation: The network layer
messages received from higher layers by placing them into datagrams
(also called packets) with a network layer header.
- Fragmentation and Reassembly: The network
layer must send messages down to the data link layer for transmission.
Some data link layer technologies have limits on the length of any message
that can be sent. If the packet that the network layer wants to send
is too large, the network layer must split the packet up, send each
piece to the data link layer, and then have pieces reassembled once
they arrive at the network layer on the destination machine. A good
example is how
this is done by the Internet Protocol.
- Error Handling and Diagnostics: Special
protocols are used at the network layer to allow devices that are logically
connected, or that are trying to route traffic, to exchange information
about the status of hosts on the network or the devices themselves.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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