In the previous section, we learned to add the DHCP server to our Raspberry Pi cluster. This was done mainly so that each Raspberry Pi in our cluster could be identified by a unique IP address. Now, as you can imagine, the bigger our cluster grows, the harder it is going to be to remember the IP address of each Raspberry Pi. Humans find it hard to remember long sequences of numbers, which is what the IP addresses are. But there is a way by which we can easily connect to different nodes without having to remember their IP addresses.

This method is called a Domain Name System (DNS). Its basic function is to translate IP addresses into words so that it becomes easier for us to remember. For example, the DNS server will translate the IP address 192.168.5.43 into raspberrypi.org and vice versa to make it easier to connect to. Indeed, this is what happens on the Internet. Each website has an IP address behind it.

When you type a URL into your browser, your computer first contacts the DNS server and asks for the IP address that it is associated with. The DNS server then searches its database for the name and returns the IP address to your computer. This whole process is called a DNS query.

Now, we can have multiple DNS servers in a single network. This way, when one DNS server doesn't have an address saved in its database, it can pass the query to the second DNS server, and so on. This is known as an iterative DNS query. This is what actually happens on the Internet. There are millions of websites on the Internet and having a single DNS server that holds a large database is not only unfeasible but also unreliable. So, there are millions of DNS servers that are distributed across the world and act together to resolve billions of translation requests from millions of clients.