If you're setting up a new webserver or another device that's going to be connected to the internet and you want to put it through its paces before it's live, or if you're having trouble with spyware and adware networks, there's an invisible file on your Mac that can help. It's called the Hosts file, and this is how to use it.
The Domain Name System
When you type in the domain name of a web site you want to visit there's a lot happening behind the scenes. Every web site, every service, almost every device connected to the Internet has a unique numeric address that tells all the other devices where it is — its TCP/IP address. The Domain Name System (DNS) translates those numeric addresses into something a bit more recognizable and memorable to humans, like, "www.imore.com" for example.
The first time you type in a web address, your Mac pings a DNS server — typically one automatically configured for you by your Internet Service Provider — to find out the TCP/IP address of the server you're trying to connect to. Your Mac builds up a hidden cache file to remember those details later on when you visit the same site again.
The Hosts file
The Domain Name System and its associated cache is your Mac's standard way of knowing how to get to where it's going on the Internet, but there's another file that can be very useful. It's called the Hosts file, and it can be used to override the default DNS information.
There are some practical reasons why you'd want to use the Hosts file instead of just letting DNS do its thing. Let's say you're testing a development server you're about to deploy, and you'd like to use its domain name instead of the machine's specific IP address. Before the system's online and accessible to anyone using DNS, you can use the Host file instead: Enter the machine's IP address and when you use that domain name, your Mac will go to that device instead.
You can also use the Hosts file to block or work around spyware and ad networks by "zeroing out" their IP addresses - putting in 0.0.0.0 then the name of the domain you'd like to block.
Lory is a renaissance woman, writing news, reviews, and how-to guides for iMore. She also fancies herself a bit of a rock star in her town and spends too much time reading comic books. If she's not typing away at her keyboard, you can probably find her at Disneyland or watching Star Wars (or both).
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