Hi,
I have just started studying for my CCNA. I have so far read about the OSI layers.Some of you may laugh but hey this is a great achievement for me.

I have question and will appreciate if the geeks can help out.

"Describe data link addresses and network addresses and identify the key differences between them".

I know the definitions of both... well sort of but am a bit confused in identifying the differences.

" List the key internet working functions of the OSI Network layer and how they are performed at a router"

one again lads, the only things so far i know about a router is that it receives data, it examines the layer 3 data to determine the destination network address. It then looks up in a table that tell it which route to use to get the date to its final destination.
Please help me understand and answer these two questions.
Much apreciated

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Layer 3 - This is a layer 3 address
eg. 200.200.200.1

- On the Internet, we use the layer
3 address to find people.

- when we want to see if a remote
station is running, we ping the
station using layer 3 address
eg. ping 200.200.200.2

- routers work at the Layer 3 OSI
because they have to use routing
tables to find the other routers.

Layer 2 - at this layer, you find the Datalink
layer. One of its function is
topology.

- it uses the MAC addresses to find
the other stations on its LAN

- it has 2 sublayers (very important
on the exam, know it)

- the Mac address is the address you
find on the network card.
eg. 00 60 00 83 DF 38

If you are using win98, run winipcfg
and you will see your MAC address
provided you have a NIC card hooked up.

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Every computer Network Interface Card (NIC) has a unique Hardware Address that is assigned to companies that make network devices. Actually they get assigned a bank of addresses to use and then when they run out, they come back for more. This is your layer 2 address. This example is not inclusive of all network adapters. Everything you will be exposed to in the CCNA, this definition will hold true.

The important thing about Layer 2 Addresses is that they are not routable (meaning that you cannot logically define networks with routers using these addresses) and for the most part, you don't get to choose your MAC Address. Because you don't get to derfine your MAC Address (some systems you can choose your MAC but that is currently inconsiquential with the current models of the OSI) there is no pattern to associate to a group of hosts on a network.

The reason why is that for you to route using MAC or layer 2 addresses, you would have to know where every MAC Address is on every device connecting networks, memory would be just a little bit of an issue with this. So MAC Addresses or Layer 2 addresses are used to identify a host within a logical network or broadcast domain.

To identify logical networks of hosts, you use a layer 3 address. This is much better since you have the ability to choose your layer 3 address to define a group of hosts in a broadcast domain. Above that you can then summarize these together (CIDR) to reduce the burden on your routing tables even more.

So to get from here to there, you use the Layer 3 address. Once you get there, to get to the right host on the network, you use the Layer 2 address.

That should be a good brief explination of this. If you want an excellent explination of how the internet works in terms like these, check out Internet Routing Architectures Cisco Press Book. The first half is an awesome description of the internet and how everything got to how it is today.

__________________
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Those two posts above I think are great. I am also preparing for this paper!, but I have gone past the OSI layers and I believe I know how it all works now.
- To add to the above two posts:
Well now we know that each host on a network has a unique MAC address to Identify it on the "local" network and that this is burned on the NIC at the factory!
- Well I think you should also know that these MAC addresses (a.k.a Datalink addresses) can be resolved to Network Logical addresses ( IP addresses) using RARP (Reverse address resolution protocol) and IP addresses resolved to MAC addresses by ARP (Address Resolution Protocol).
- Routers work at the Network layer of the OSI model.
- Switches and Bridges work at the Datalink layer of the OSI model.
-Network layer encapsultion type is called Packets or Datagrams while Datalink type are Frames.( Transport layer = Segments, Physical= bits)

__________________
"No Work is done in vain!
The only place where Reward comes before
Work is in the Dictionary"

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I guess I understand the differences now. Thanks Guys.

Appreciate your valuable inputs help

Rosh

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Hi,
Go through following link. I think you will be satisfied.
http://www.how2pass.com/CCNA/study_...l/osilayers.htm

A.A. Shah

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