Thursday, November 18, 2010

Address Resolution Protocol

ARP stands for Address Resolution Protocol. It is used to determine a physical address or MAC address from a IP address that is known. The router or switch will then broadcast the incoming request and asks to the users "who has this IP address?". If the client does not have that address, it will ignore the packet. If the client does use that IP address, if will then reply with an answer "I have this IP address, and my MAC address is aa:bb:cc:dd:ee:ff".

A simple illustration can be explained using a house's phone number and house address. A phone number of a house can be changed. This is also the same for an IP address of a client, it can be changed easily. A house address on the other hand is physically attached to a certain location that will be always there. This is the same as MAC or physical address of the client. So, when ARP is sent, a request is done to trace a phone number and the operator (Switch) is required to find out the home address of that phone number. The operator will then knock each door simultaneously and asks to the resident whether the given phone number is their phone number.

There is also RARP which stands for reverse ARP. It used used reversely when a device want to know the IP address of a device but has only the MAC address of it. So basically it will ask to a certain home address its telephone number.

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