Where do you use domain names?

I realize the domain name system isn’t an easy concept to understand. It took bright computer geeks years to perfect it to the point we see it today, and although it works pretty well, the volunteers who maintain the Internet are still tweaking different parts to ensure that we’ll have enough domain names for the foreseeable future.

However, it’s important to have a basic grasp of what is going on with domain names because you simply can’t avoid them in the Internet. They’re everywhere, and you use them constantly everyday, whenever you send email or connect to a web site.

In fact, that’s where we’re going to turn our attention in the next archive since domain names play a huge part in forming email addresses and the URLs (Uniform Resource Locators) that identify web pages.

How to read Domain

So, in postoffice.yahoomail.yahoo.net, the top level domain of net identifies the machine as belonging to an Internet Service provider, or ISP. Obviously, since there are a lot of other ISPs in the U.S., many other machines exist in the net Top-level domain as well.

That’s where the middle-level domains come in handy. In our sample, yahoo is the next part of the domain name (remember, we’re working backward, getting more specific as we go). Middle-level domains help differentiate all the different computers in the top-level domains. So, the word yahoo in postoffice.yahoomail.yahoo.net says that this computer is run by Yahoo. The next middle-level domain, yahoomail, makes it clear that the computer in question is part of Yahoo Service.

Domain name only require a top-level domain and a second-level domain, but there’ no limit on how many middle-level domains a full domain name can have (well, there probably is some limit, but it’s not the sort of thing normal people give a hoot about). You distinguish between middle-level domains by looking at their position in the domain name. The middle-level domain that comes in front of the top-level domain is called the second-level domain, the one before that is the third-level domain, and so on.

We still haven’t uniquely identified the postoffice machine. The remaining part of the domain name, postoffice in postoffice.yahoomail.yahoo.net, is the machine name, and uniquely differentiates that computer from all of the other computers that Yahoo runs.

Now think for a minute about how the domain name uniquely identifies this computer. If you change any part of our sample domain name, you’re referring to a different computer. And, if you change the .net top-level domain, you could be referring to a computer run by a university, government, agency, or non-profit that also has the initials Yahoo.

As a minor twist, domain names are sometimes purely geographical. For instance, a friend of mind runs a computer whose full domain name is memory.ithaca.ny.us. In this case, the top-level domain, ny, puts it in New York State, and the third-level domain, Ithaca, narrows it further to the city of Ithaca. Finally, the machine name, memory, differentiates the computer from all the other computers in Ithaca, New York.

Think about your snail mail address for a moment if you’re having trouble getting your mind around the levels of domain name. Your address identifies your house, your street, your city, your state or province, and your country. Just as each domain encompasses those before it, so does each part of your address encompass the previous parts. The state or province is inside the country, the city is inside the state or province, ad the street is inside the city.

The HIERARCHY OF DOMAIN NAMES

Starting with the top of the the hierarchy, which sits at the end of the domain name, you see the top-level domain of net. There are a small number of top-level domains on the Internet, and they fall into two categories-topical and geographic. Topical top-level domains identify the type of organization that runs the computer in question, and are used mostly in the United States. Geographic top-level domains identify the country where that computer lives, and used primarily outside of the United States. The first table shoes the main topical top-level domains(more may be coming soon, since .com in is filling up) and the second table gives some examples of common geographical top-level domains. Every country has its own top-level domain country code.

Top-level Domain	Organization Type
.com .edu .gov .mil .net .org	Commercial Educational US Government US military Network (ISPs) Non-profit organization

Top-level Domain	Country
.au .br .ca .de .fr .jp .uk .us	Australia Brazil Canada Germany France Japan United Kingdom United States