DNS study notes — Name resolution


In this post we will cover :

  • Name resolution overview and history
  • Domain

What exactly is name resolution?

In TCP/IP, it is the IP address that identifies the communication partner, but it is difficult for humans to remember the enumeration of numbers. We want a mechanism that allows you to specify a communication partner with a character string (domain name) that is easy for humans to remember. Under this mechanism, it is necessary to associate an IP address with a domain name, and this mechanism is called name resolution. Under name resolution, the user does not need to be aware of the IP address.

  • You don’t have to remember your IP address.
  • If only the domain name is fixed, it is not necessary to inform the user when the IP address is changed.

One of the name resolutions is DNS (Domain Name System). The simple mechanism of DNS is that the client inquires about the database that corresponds to the IP address and domain name .

Non-DNS name resolution

Hosts file

In the past, name resolution was performed using a list Hosts file that corresponds the host name and IP address .

Problems with name resolution by Hosts file

  • It is difficult to ensure that every host on the network has the latest Hosts file.
  • The number of hosts increases, the Hosts file grows, and traffic increases.

Name resolution by DNS

Name information is collectively managed by the name server. The client queries the name server each time it communicates. You can manage host information within your organization. Even if you register or change the host name or IP address, it will be completed by the processing within the organization.

Domain namespace

Under DNS, the host name (the device name of the device that has a name on the network) and the domain name (the name of the organization that manages the host) are managed in a tree structure.

  • root is the root domain
  • node is the domain name
  • leaf (terminal node) is the host name

The tree structure makes it possible to find all hosts by tracing from root. The DNS tree is called the Domain Name Space . A domain is subject to DNS only after the domain name is registered in the domain namespace. Nodes under a domain cannot have the same name in the same hierarchy. In other words, the domain namespace needs to be managed in the same hierarchy so that domain names are not duplicated.

Domain management

Domain names on the Internet are managed by ICANN (Internet Corporation for Assigned Names and Numbers) . ICANN has transferred management of the domain directly under the root domain to the registry .When an organization actually acquires a domain name, it asks a registrar to register it.


The domain managed by the registry is called the Top Level Domain (TLD), and there are the following types of TLDs.

  • Country Code TLD, ccTLD (2-letter TLD that can only be used by domestic organizations, with the exception of the United States)
  • Generic Code TLD, gTLD ( TLD with 3 or more characters that can be used regardless of the country to which you belong)
  • Only in the United States, you can use a TLD of 3 or more letters, which means the form of an organization.

The domain directly under the TLD is called the Second Level Domain (SLD) and represents the attributes of the organization .


Hosts directly under the same domain may be able to specify the destination only by the host name. The notation of the domain that describes all domain names from the root domain is called Fully Qualified Domain Name, FQDN .

 by the author.



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