hi all,

Have you ever wondered that why there is two level of addressing in OSI model?
If IP address can identify any machine acrosss networks then what's the need of Physical Address. Theoretically Data Link layer transfers data to adjacent node by using physical address, but that can also be done by using IP address.
Have you ever pondered about that point?

Here's what i think.
Initially computers were connected using various different LAN topologies like token ring , ibm sna , ethernet, etc
All these tech. correspond to data link layer and had there own addresses [ mac addrress in case of ethernet.]
Now these networks using diff. lan tech. were incompatible with each other ie. u can't interconnect a network using token ring to a network with ethernet because both uses diff. adressing scheme to identify computers on lan.

To solve this problem a neutral adressing scheme was required which was to be independant of lan tech . and sit above icompatible lan tech. to interconnent them.
Thats why tcp/ip layer was introduced with ip address as a means to identify any end system in whole internet.

Thus it can be said that Physical address is always needed but IP address may not be needed always.
Though the above statement is hard to believe because TCP/IP widely used on every device that does some communication.

Do leave your comments.
Regards.

 

 

 

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Well the MAC Address is used to relate the IP Address to a physical computer. Each ethernet device has a unique MAC address, but any computer can have any random IP address. But when you map a IP Address with the MAC Address, you know exactly what computer is using that IP Address.

Also think in terms of network security. If there wasn't a MAC Address, there couldn't be any physical evidence to prove you used a IP address for questionable actions.

 

 

 

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It pretty much goes like saying that if you tell a router to forward a port to an ip in your local network and there were no MAC addresses, it wouldn't know where to go. The ip is like an identification number that is bound to the MAC address.

Hope that makes it clear, also as snutz said.. there will be no way to know if you've used an ip for some illegal activity for instance. Even though your ISP can track the ip down to one certain house, say there is more than one pc and person living there. They have no idea which person did it. (Unless they had a peek in the routers logs etc..)

-HellFire

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The key idea in OSI model is that each layer can work without any regard of the lower layers. If the lower level addressign were used, the upper level wouldn't satisfy the condition. Ie. it would need lower layer information.

If you used only physical layer addressing in the Internet the whole network would have to use same kind of devices or have very complicated address mapping systems. Either way it wouldn't just work. Using same kind of deviced for client access and backbone networks would be either inefficient or way too expensive. The mapping would basically be a mess and most likely very time consuming operation.


Also IP address contains more information than the MAC address. Mac address is just a unique identifier, kind of like your social security number. No one has the same social security number as you. Well we don't know if there are NICs with same MAC addresses... There shouldn't be at least. IP on the other hand has routing information in it. It's kind of your home's address. The address is unique at least in your city, possibly entire world, but it also contains rough information how to get to you. What I'm talking about is subnetting, you get the subnet from the address and you've narrowed the possible locations hugely. In same way, if you want to get to someones real address you probably check the city, province and country first and first head there. IP addressing is a system that is possible to handle by distributing it to several locations. If we used just MAC addresses in the Internet we would have to have a huge central hub that would route all traffic. That just isn't possible with our technology and even if it were it wouldn't be sensible.

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This problem actually occurs in the N/w layers. In case of ARP, a router has 2 IP's-one for the local ethernet and the other for the FDDI ring.
SO to resolve the hosts exactly, physical addresses are quintessential. We can find various solutions for it such as using a proxy ARP and also to have a host immediately see the dest'n on a remote N/w and just send all such traffic to adefault ethernet address that handles all remote traffic.

If you are confused, relax and take a chill pill

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