Let’s start with the basics, the basis of the system. DNS stands for Domain Name Service.
What is DNS?
It’s like the whitepage directory for the Internet. You supply a name, DNS supplies a number. The name in this case is specifically a hostname and the number is an IP address. Without DNS you would have to remember every IP address of every website you want to visit. With over 300 million websites on the web, remembering every IP address would be utterly impossible, well unless maybe you’re Kim Peek: The Real Rain Man.
What’s a hostname and what is a domain name?
The term hostname refers to the unique part that identifies a host on the Internet. In www.no-ip.com, www is the hostname. In that same example, no-ip.com is the domain name. The host portion prepended to the domain name is often referred to as the hostname and that is how we use the term here. So, in summary, www is the host, no-ip.com is the domain, and www.no-ip.com is the hostname.
What’s an IP Address?
An IP address is a unique number that allows computers to locate each other on a network. The Internet is a big network and uses IP addresses to route communication to the proper host. The IP in IP address stands for Internet Protocol. An IP address looks like this: 126.96.36.199.
So wait, how does this work?
Remember, DNS looks up hostnames and returns IP addresses. When you type in a hostname, say www.no-ip.com, your computer checks with a DNS server provided by your ISP to find the IP address for that hostname. Your ISP may already know the IP address for www.no-ip.com and it will just reply with it, but if it doesn’t, it knows how to find it on the Internet. First, it will ask special servers that already have IP addresses for for the authority of no-ip.com. These special servers are called the root servers. The root servers know what IP address has the information your ISP is looking for. Then it is just a matter of your ISP requesting the IP address and returning the results to your computer.
Here is a analogy. Imagine you live in California and you want to find my phone number in New York. Your phone book couldn’t possibly contain all the possible names and numbers that you may want to look up, but it could contain a number to call to do lookups for New York. You consult your phone book and find New York’s number and call it, asking for my number. New York is a big place. So it breaks down the lookups by county. It knows what county I’m in and get the number and returns it to you and no one has to carry around every single name to phone number mapping in the world.
And IP addresses can be dynamic?
Imagine that once a day at a random time your phone company changed your telephone number. If there was no way to look it up no one would be able to call you! This is what a dynamic IP address effectively is. It’s a changing phone number. The number used to find your computer on the Internet can be different each time you log on, or at the discretion of your ISP. Dynamic DNS is like a live phone book that updates itself when your number changes. It knows your number at any given time so that people can connect to you. To make sure the dynamic DNS server has your proper IP address, your computer must run an update client to let the server know when your IP address has changed.
A real example, please?
Alice has a computer with a dynamic IP address running a webserver. Her friend Bob wants to see her web page. Alice tells Bob that the hostname of her computer is alice.no-ip.com. Bob types into his browser, http://alice.no-ip.com/. His computer contacts his ISP to get the number for alice.no-ip.com. His ISP’s DNS server doesn’t know that one, so it checks with the root servers. The root servers tells Bob’s ISP the IP address of No-IP’s nameservers and his ISP looks up alice.no-ip.com there. He receives the number and connects to Alice’s web page.
Now, to be sure Bob got the right number Alice has an update client provided by No-IP running on her computer. This update client informs No-IP.com when her IP address changes so that Bob will get the right number. Alice’s often changing Internet address, her IP address, can always be found by Bob when he uses alice.no-ip.com as her hostname.