Primer 2: The Yellow Pages (DNS)

This primer series is meant to be a less technical discussion around modern security tools and techniques. This may not match the otherwise technical nature of this site.

A Quick Reminder of the Phone System

To call somebody, you’ll often times click a contact today. If you’re ordering a pizza or calling somebody you don’t often, you may dial a number.

Even these days, a phone book or “Yellow Pages” is often found in mailboxes and at the end of driveways worldwide. It contains a ton of paid-for listings for businesses, and also a “White Pages” section which contains names of people. This information is often times alongside an address and phone number, and perhaps what the organization does.

This reference book is what enables you to talk to a person easily, without needing to memorize the numbers. You have a lookup table to find it.

What is an IP Address?

An IP Address is a temporary numeric identification for your computer on a network. Similar to an address, however an IP address doesn’t necessarily tie you to a location (despite what movies say). Often times the information may simply point to your town or a town near you (if it is correct at all).


Physical AddressIP Address
Robert Lerner
123 Main Street
Anytown, XY 10203

If I want to mail you a letter or package, I’d put your address on it and the mail service would take it from there (if you pay them, of course!)

When two computers talk to each other, they use the IP Address to go out to the internet and say “Hey, I’m looking for, who has that?”

There is a DNS Server that replies “Hey! I know, that website is located at, and the TTL is 3,600 seconds!”

Then, your computer will remember this number and website for 3,600 seconds (one hour). After one hour, the “Time To Live” or TTL will expire, and your computer will make sure the number is still accurate. This information is cached to reduce burden on the DNS Servers and reply to you faster.

Just like using the Yellow Pages to look up a business, you simply enter “” into a browser, and the system will look in the DNS for the right place to call.

DNS System History

A very long time ago when the internet was still new, there were not very many websites to visit. The solution at this time was to add the information to a file called the “hosts” file, which looked similar to this:

Because this file was easy to maintain at the time, it was considered “good enough.” Eventually, as sites were added the lack of sustainability was obvious. There was no good way to circulate large hosts file to every user of the internet. It was that moment that the “Domain Name System”, or “DNS” was made.

How Does DNS Know?

Now, instead of a list that is circulated and easily modifiable, site owners have a registration for their domains that only they can change. Those owners will teach the DNS servers that their site resides at a specific address. Other DNS servers will copy that information and store it until you access it.

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Robert Lerner

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