Need for IP address

Why do we need an IP address when the MAC address is unique? Cant we communicate only with开发者_如何学编程 the MAC address?

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You COULD communicate using only the MAC address, but only on your local network. IP addresses are routeable, without every system on the network needing to know about every other. You just need to know a range of addresses that are on your local network, and throw everything else up to your router. The same thing happens at the ISP level. "All 216.x.x.x traffic goes that way, all 105.x.x.x goes that way..."(Obviously a gross oversimplification, but that's the basic process).

If we tried to route everything by MAC address, every machine on the network would have to maintain a list of every other participant, and it just wouldn't scale.

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No. MAC addresses are specific to Ethernet, IP is independent of the underlying hardware. You can connect machines that don't use Ethernet to the Internet, if you have the required bridges.

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• MAC addresses are not unique. MAC addresses are reused between media. This is why wireless (802.11) and wired (802.3) may not both be present on one collision domain (see 802.1D).
• MAC addresses are not clustered -- meaning that devices which are nearby in network space do not have nearby MAC addresses. IP addresses do have this property of locality. Do you intend to route packets by having a universal list of MAC addresses copied to every computer on the Internet, or do you intend to route packets to their destinations through a hierarchy of localities?
• On a single collision domain, MAC addresses can be the primary addressing mode (q.v. arp and rarp). However, extension to multiple collision domains is ineffective for the above reasons.

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A great professor of mine named George Varghese, now at UCSD, made the following apt analogy: You want to send someone a letter. The analogy of sending to a device anywhere in the USA based on its MAC address is like sending someone a letter knowing only their Social Security Number. It does uniquely identify someone (OK, yes, SSN isn't guaranteed unique, but suppose it was for the sake of example), but it would be very hard to find them without some giant table of where everyone lived that you could look up indexed by their SSN.

An IP address (and the similar Open Systems Interconnect, or OSI, network addresses) are more like USA phone numbers with area codes and exchange numbers: (AAA) BBB-CCCC, where AAA is an area code, BBB is an exchange number, and CCCC identifies an individual line at that exchange. There is hierarchical information encoded in that number, so that when you are far away from the destination, you only need a small table indexed by area code to determine a good "next hop" to forward the call to, rather than a table of all phone numbers in the country.

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Ethernet is a Medium Access Layer protocol. It was designed specifically to connect computers on the same network. If you want to connect computers remotely located, you certainly need to jump to destination by hopping through several routers. IP (Internet Protocol) was designed with this goal in mind, hence the need for it, while Ethernet protocol does not support routing. Only some forms of primitive bridging that would not scale for something huge like the Internet.

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they are used for different protocol layer.

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MAC address is your device specific address. It has no relation with the geographical location, etc. you are in currently.

Ex: You can buy a cellphone/laptop in US and use it in Japan, Australia, etc. But MAC address would remain the same. But IP address would change with respect to the network you are connected to.

So it is difficult to route packet in an internetwork of portable devices especially.

How would it be: Consider you have a portable network-accessing device with you on which you are using the internet. If we use only the MAC address, how would any incoming packet find the location of your portable-device. Since MAC address gives you only a fixed 48-bit device address. (The worst case scenario is using a desktop computer and having a MAC address without the IP facility. Coupling it with the static table to find your predefined location based on the MAC address, but our life is incomplete without these portable devices right?)

Thus we need some addressing scheme that can help us with addressing in a big and portable environment like internet, and thus the role of IP comes into picture, where address is hierarchal to provide a more geographically exact location.