When you want to call someone on your cell phone, you likely just find the person in your list of contacts and select their name. The handset then uses the unique phone number associated with that person and makes the call. In general, you don’t need to remember the phone number itself, as the address book stores it for you under the name of the contact.
The Internet works in a very similar way. Each server on the Internet has an address assigned to it and a name. There is a global address book which stores the address (or addresses) along with the associated name (or names). This huge address book is known as DNS (Domain Name System). The way it works is that when you type a URL into your browser, e.g. maketecheasier.com, then the browser (via the underlying operating system) queries DNS to get the address for the server which hosts the web site. A similar, but not identical, thing happens when we send emails.
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Advanced Guide to nslookup (Make Tech Easier)