Although the IP address is an address that is an indispensable address (name) for computers in a network, it is difficult to remember as it is composed of only a set of numbers, and in addition to this, it is also difficult to make a correspondence with the real world.

In the real world, a typical name that is strictly controlled is a residence address.
For example, the address of Kumamoto University can be expressed by the characters below.

To express this only using text, the figure above becomes as shown below.

The order is in the way used in the Western World, in which the larger location category comes later (right side).

Similar to the rule for residence addresses, there is a system for giving a name that is easy for people to understand to a network or computers on a network, and it is called the DNS (Domain Name System).

See below for an example of giving a name to the computer using the DNS.

The figure below graphically indicates the name above.

We can imagine that it represents Kumamoto University (kumamoto-u) in Japan (jp) from the name. For your information, the IP address of the server above is 133.95.34.20, which is like its real name, and mail.st.kumamoto-u-ac.jp is like its nickname. As described above, the name given by the DNS is called FQDN (Fully Qualified Domain Name).

In the explanation above the FQDN is described as being similar to a nickname, however, the DNS is strictly controlled in the world in a similar way to the IP address, therefore we cannot give the nickname by ourselves. The reason for this is that it is a system to control the other names of IP addresses, which is strictly controlled. Thus, if a name that is not controlled is given to an IP address, we cannot communicate using the IP address.

For instance, let’s assume that you are trying to display a URL, which is http://el.kumamoto-u.ac.jp/. However, the web browser (or the PC you are working on) does not know the IP address (real name) of the computer represented by the FQDN (the other name) of www.el.kumamoto-u.ac.jp. In addition to this, the computer (PC) cannot directly communicate using FQDN, it always needs to know the IP address of the host in order to communicate with it.

For this reason, generally PCs ask for the IP address (which is the real name) of the computer represented by FQDN, to a special server that controls the DNS, which is called the DNS server. By doing this, communication with the host that it was originally required to communicate with is made possible.

The operation can be understood using the figure below.

(Note) To make the system above work, you have to specify the DNS server you use when you set up the network on your PC. In fact, when the IP address is set automatically by DHCP, the DNS server is also set automatically.