Practice English Speaking&Listening with: What is ARP (Address Resolution Protocol)?

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Sabrina Polin: Address Resolution Protocol, or

ARP, is a procedure for tracking an IP address

back to a device in a local area network. This

device has a unique physical address which is also

referred to as a media access control address, or

MAC address. ARP works between layers two and

three of the OSI model: the network layer, where

the MAC address lives, and the data link layer,

where the IP address lives. ARP's job is to

translate 32 bit addresses into 48 bit addresses

and vice versa. This translation is necessary

because IPv4 uses IP addresses with 32 bits will

MAC addresses use 48 bit addresses. The mapping

process begins when a packet, or unit of data,

arrives at a host gateway. The packet's arrival

prompts the ARP program to match the IP address

with the MAC address. The host searches its ARP

cache for a pre-existing translation for that

packet, and if it doesn't exist already, performs

the ARP process. For example, let's say computer 1

needs to send a packet to computer 2 in an office

on the same LAN. Computer 1 needs to determine a

MAC address for computer 2. Because computer 1

doesn't already have the address and its ARP cache,

it uses ARP to translate computer 2's IP address

into a MAC address. Once it has the MAC address,

computer 1 stores the information and it's ARP

cache and can then send the packet. Since ARP only

translates 32 bit IP addresses to 48 bit MAC

addresses, it's only good for IPv4. When packets

are sent with IPv6, the Neighbor Discovery

Protocol is used instead.

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