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.