What is ARP

What is ARP

　In a LAN, communication is performed using a mechanism called Ethernet. Here, we will explain how communication is actually performed using IP addresses and MAC addresses.

Here’s a recap.

IP address — logical address
MAC address — physical address

is called

　An IP address is an address arbitrarily set by humans for convenience to facilitate grouping of computers participating in a certain network.

　If you look at it from another computer, you can’t tell which IP address has been assigned. Also, when computers are moved to different networks, their IP addresses change to make it easier for humans to group them.

　In this way, with only the IP address, it is not possible to know which IP address has been assigned to the communicating party, and neither does the communicating party know its own IP address.

　A MAC address is a unique number assigned to hardware. This number is written in ROM at the time of manufacture and cannot be changed in principle. (*This can be changed.)

This MAC address will not change even if you move to another network.

　Ethernet has a mechanism to identify the communication partner by using two addresses, this IP address and MAC address.

　This mechanism is realized by a protocol called ARP (Address Resolution Protocol). ARP is a protocol used in TCP/IP networks to obtain a MAC address from an IP address.

* Confusingly, RARP (Reverse Address Resolution Protocol) also exists. This is a protocol that obtains an IP address based on a MAC address. Because of the confusion, ARP and RARP are used together in many certification exams.

Here’s an analogy for what ARP does:

　For example, I made an appointment to meet for the first time with “Ako-san”, an e-mail friend with whom I had only communicated via e-mail, and decided to meet in front of the station at 12:00. The two will meet for the first time. I have only exchanged texts with him by e-mail, and I do not know his face. All I know is her name.

　You went to the meeting station at 12 o’clock. There was a large crowd in front of the station. How do you find someone to meet up with?

If you don’t do anything, you won’t be able to recognize “Mr. A” no matter how long you wait.

What would happen if you yelled, “A-ko-san!”

If “Mr. A” is there, “Mr. A” should be able to reply!

In this way, you can find “Mr. A” whom you only know by name.

It is ARP that shouts out to everyone and finds the other party.

How ARP works

　An IP address is an address arbitrarily set by humans for convenience to facilitate grouping of computers participating in a certain network.

　If you look at it from another computer, you can’t tell which IP address has been assigned. In other words, even if you know the IP address of the person you want to communicate with, you don’t know which computer has that IP address.

　For example, when computer A communicates with computer E, even if you know the IP address of computer E, you cannot know which computer E is.

Therefore, computer A will, for all computers on the network,

computer E?

query.

This is called broadcasting.

Ignore computer B, computer C, and computer D because they are not addressed to you.

Since the inquiry is addressed to itself, computer E returns a reply to computer A.

Thus computer A can find computer E.

　An inquiry (broadcast) made by computer A to all computers is called an ARP request. Also, when computer E replies to computer A, it is called an ARP response.