On the Internet, the equivalent to a telephone directory is the DNS (Domain Name System). DNS is a network of servers that translate numeric IP addresses into readable Internet addresses, and vice versa. DNS takes the friendly domain name you type when you want to see a particular website, such as www.example.com, and finds the equivalent IP address, such as 203.0.113.2. Network devices need the actual IP address to find the website, but domain names are much easier for users to type and remember than IP addresses.
A DNS server is a server that performs this translation. Many organizations have a private DNS server in their network that responds to DNS requests. You can also use a DNS server on your external network, such as a DNS server provided by your ISP (Internet Service Provider).