|  | 15.2 | Domain Name System
| 15.2.1 | Problems with using IP addresses

In the network layer chapter, you learned that the Internet is built on a hierarchical addressing scheme. This allows for routing that is based on classes of addresses, as opposed to individual addresses. The problem this creates for the user is associating the correct address with the Internet site. The only difference between the address 198.151.11.12 and 198.151.11.21 is one transposed digit. It is very easy to forget an address to a particular site, because there is nothing to associate the contents of the site with its address.

In order to associate the contents of the site with its address, a domain naming system was developed. A domain is a group of computers that are associated by their geographical location or their business type. A domain name is a string of characters and/or numbers, usually a name or abbreviation, that represents the numeric address of an Internet site. There are more than 200 top-level domains on the Internet, examples of which include the following:

> .us - United States
> .uk - United Kingdom

There are also generic names, examples of which include the following:

> .edu - educational sites
> .com - commercial sites
> .gov - government sites
> .org - non-profit sites
> .net - network service

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DNS: The Domain Name System