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MAC Address Question

Discussion in 'Routing & Switching' started by Crazydave1990, Apr 25, 2012.

  1. Crazydave1990

    Crazydave1990 Bit Poster

    Hey guys, me again, if some of you have seen my previous threads, I'm a bit of a n00b to networking, working towards getting a CCNA, if you've not seen any previous threads, well, I'm a n00b to networking haha!

    anyways guys, I'm reading through the OSI Layer, and well, I'm slightly confused... MAC and IP, they're both used for addressing, but why do we need a MAC address? what does it do? I know an IP is a layer 3 function, Mac is Layer 2, but what is the difference? IP is internet protocol, so an address for sending and recieving data over the internet between destination and source...

    any advise would be appreciated guys, thank you.
  2. dmarsh

    dmarsh Terabyte Poster

    Read up on ARP. MAC is a lower level method of addressing, it is relatively fixed, manufacturers tend to get assigned MAC address ranges. The idea is it is burned into the NIC in the factory and so identifies hardware uniquely in the world. There are times when you won't have an IP address, ARP broadcasts can then be used on a subnet in order to assign one.

    IP addresses are a higher level abstraction, they are used for general routing and switching and are not limited to the local subnet like most MAC address uses.

    An IP address can change over time, for example most on most clients it is generally assigned for the period of a DHCP lease. It it not tied to any physical device and does not relate to the data-link layer. There are non IP protocols that can operate over Ethernet hardware with MAC addresses but that do not use IP addresses at Layer 3.

    MAC address - Wikipedia, the free encyclopedia
    How ARP Works
    Last edited: Apr 26, 2012
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  3. Coupe2T

    Coupe2T Megabyte Poster

    Yep, As dmarsh say, IP Addresses are not permanent, they are assigned. Think of it this way, when you first connect a computer to a network, how does it get an IP address? It doesn't know any IP addresses within the network it's connected to, so it doesn't know where to get an IP address of it's own from etc.

    The MAC address is physically burned in so the computer can send out it's static MAC address and say I need to connect to this network, and it means that the network can then respond to that hardware address. If the Computer did not have a MAC address and it connected to the network then it would have no way of giving it's own address for the network to be able to communicate and assign an IP address etc.

    The above is a fairly high level overview but you get the idea. It's an initial way of being able to communicate to retrieve an IP address and then be able to send IP packets etc across the network.
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