If a MAC address is unique, then why cannot we use MAC address instead of IP address to define a host in a network.

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IP address
MAC address
Networking
If a MAC address is unique, then why cannot we use MAC address instead of IP address to define a host in a network.

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Hello Bubai,
first the mac address is considered only at the data link while the IP address is at the network layer

An IP address and subnet mask are used together to create a hierarchy for routing. The classic example is of the US Postal Service. Your address may be 123 Cow Road, timbuktu LA 12345. The postal service will route the mail to a regional center, then a local post office, and then finally to your house based on the address.

If you were given the address space 172.16.1.0 255.255.0.0 (and it weren’t private address space) you would have 16 bits of address space to assign to your hosts. Routers would not need to know how to reach every possible host within that address space, only how to reach that address space itself.

With a MAC address, this is entirely impossible. An organization that creates NICs or any ethernet interface for that matter is given a block of MAC addresses that they distribute as they please. This means that devices that fall under the same block of MAC address space could be distributed anywhere throughout the world. This creates a completely un-scalable model that would be impossible to route. Every router on the internet would have to house information on every possible endpoint. Even if every endpoint in the entire internet were in a static location, it would be impossible for any of today’s routers to handle.

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  • Technochic
    Excellent points all! The other point is that a MAC address is static and hardcoded to the NIC on the PC and therefore cannot be re-assigned as necessary. Defining a host by IP address is far more flexible of course.
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  • petkoa
    In IPv6 MAC is used as a component of the unique host address: it consists of a network portion used for routing, which is passed to the host by DHCP server, and a "MAC portion" appended by the host self and broadcasted to the LAN. I'd say, risking to be condemned by IPv6 profets, a situation not very much different from the current IPv4... BR, Petko
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  • BlankReg
    You should also realise that a mac address is not as fixed, and not as unique as you may think. Although we dont use it these days, DecNet used to do exactly what IP now does, define a network and host id, but used the mac address to do this, by changing it's value to incorporate these. Routing was therefore done at layer 2, and not at layer 3 as we do now. If you use a packet sniffer to generate packets, these can go out with any mac address you want, not just the one of the host running the software. You can also change the mac address of many devices, if you should want to, through the software. Usually you do not want to, but the ability is there. Other than that, the comments made by the others on here are all valid, and IP is a much more flexible and easier protocol to deal with, which is why it survived, and DecNet did not (even Dec stopped using it and moved to OSI !).
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