Definition: Domain Name Service or DNS means the aliasing of IP addresses to domain name addresses and the reverse resolution of a domain name address to the corresponding IP address in order to support website visitors with "readable" addresses. Domain name servers are involved in the encryption and decryption of IP addresses.
Every website has an IP address under which it can be reached. IP addresses consist of numbers that are divided into blocks (0-255). An example would be 123.456.789.000. These are IPv4 addresses consisting of 4 blocks (also known as octets). However, IPv6 has already been developed, in which the IP addresses are divided into 8 blocks consisting of hexadecimal numbers (0-9, A-F). An example of this would be 2f13:00ad:ee21:0124:afd4:ff67:781b:cd00. The reason for this is that many billions of systems will receive IP addresses in the future, such as coffee machines with an Internet connection, mobile devices and much more. IPv6 will be able to handle this enormous number.
If websites could only be accessed via their IP addresses such as 134.119.39.85 (corresponds to arocom.de) or 91.198.174.192 (corresponds to wikipedia.org), this would not be user-friendly, as people find it difficult to remember such series of numbers. In addition, if the IP address of a website changes, it would no longer be possible to access it via the old IP address.
To make surfing the internet easier, the Domain Name Service, or DNS for short, was developed in 1983, which allows websites to be given "letter addresses" (source: https://de.wikipedia.org/wiki/Domain_Name_System). The IP addresses are encoded behind readable and "memorable" series of letters (DNS address) and can be broken down again into IP addresses.
If you are interested in finding out which IP address belongs to a particular website, you can check this here, for example: https://ip-lookup.net
Structure of DNS addresses
A DNS address such as www.arocom.de consists of 3 parts and is read from right to left (or from back to front). First there is the Top Level Domain (TLD) "de". This is a so-called Country Code Top Level Domain in contrast to "info" or "org", for example. The TLD is the first visible hierarchy level of the DNS address. "arocom" is the second part and is called the Second Level Domain (SLD). The third part is the https://de.wikipedia.org/wiki/Domain_Name_System. For web addresses this is often "www", for mail servers it could be "mail". When registering a DNS address, a DNS server (or name server) is also specified, which manages this DNS address.
What happens when a DNS address is called up?
If, for example, the address www.arocom.de is called up, the browser performs 4 cache queries: The browser first checks its own cache. If the address has been called up before and is still in the cache, the browser can open the page directly.
If this is not the case, the browser checks the cache of the operating system, as this also stores IP addresses.
Only if no data is available at computer level does the browser ask the router whether this data is in the router cache.
If this is not the case, the router queries the ISP (Internet Service Provider). This provider has also saved web addresses.
If these 4 caches do not return any results, a query chain is started by the provider's Domain Name Service to find out the IP address. It makes a request to the root server, which reads out the TLD "de" and informs the ISP DNS server of this.
The ISP DNS server will now contact the name server of the TLD "de" and ask there what the IP address of www.arocom.de is. As this is not usually responsible for this DNS address, as it was not specified during registration, it will inform the ISP DNS server of the name server responsible for the domain "arocom.de".
If the ISP DNS server now queries the DNS server for the domain arocom.de, it will respond with the IP address as it is responsible (authoritative) for the domain. In addition, the ISP DNS server, router and browser store this information for subsequent calls to the DNS address www.arocom.de.
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