DNS stands for Domain Name Server, the domain name service used to link IP addresses with domain names. DNS is a globally-distributed database of mappings between hostnames and IP addresses. It is an open and hierarchical system with many organizations choosing to run their own DNS servers.

The DNS protocol is used for different purposes. The most common are:

• Names resolution: Given the complete name of a host, it can obtain its IP address.
• Reverse address resolution: It is the reverse mechanism to the previous one. It can, given an IP address, obtain the name associated with it.
• Mail servers resolution: Given a mail server domain name (for example, gmail.com), it can obtain the server through which communication is performed (for example, gmail-smtp-in.l.google.com).

DNS is also a protocol that devices use to query DNS servers for resolving hostnames to IP addresses (and vice-versa). The nslookup tool comes with most Linux and Windows systems, and it lets us query DNS on the command line. Here, we determined that the python.org host has the IPv4 address 23.253.135.79:

$ nslookup python.org

This is the address resolution for the python.org domain: