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TCP/IP Network Interface / Internet "Layer Connection" Protocols
The second layer of the OSI
Reference Model is the data
link layer; it corresponds to the TCP/IP
network interface layer. It is there that
most LAN, WAN and WLAN technologies are defined, such as Ethernet and
IEEE 802.11. The third layer is the network
layer, also called the internet layer
in the TCP/IP model, where internetworking protocols are defined, the
most notable being the Internet
Protocol. These two layers are intimately
related, because messages sent at the network layer must be carried
over individual physical networks at the data link layer. They perform
different tasks but as neighbors in the protocol stack, must cooperate
with each other.
There is a set of protocols that
serves the important task of linking together these two layers and allowing
them to work together. The problem with them is deciding where exactly
they should live! They are sort of the black sheep of the
networking worldnobody denies their importance, but they always
think they belong in the other guy's layer. For example,
since these protocols pass data on layer two networks, the folks who
deal with layer two technologies say they belong at layer three. But
those who work with layer three protocols consider these low level
protocols that provide services to layer three, and hence put them as
part of layer two.
So where do they go? Well, to some
extent it doesn't really matter. Even if they are black sheep
I consider them somewhat special, so I gave them their own home. Welcome
to networking layer limbo, also known as OSI layer
This is where a couple of protocols are described that serve as glue
between the data link and network layers. The main job performed here
is address resolution, or providing mappings between layer two and layer
three addresses. This resolution can be done in either direction, and
is represented by the two TCP/IP protocols ARP and RARP (which, despite
their similarities, are used for rather different purposes in practice.)
Background Information: I suggest familiarity with the basics of layer two and layer three before proceeding here. In particular, some understanding of IP addressing is helpful, though not strictly necessary. In general, if you are going to read about IP anyway, you would be better off covering that material before proceeding to this section.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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