15.2 Domain Name System 15.2.1 Problems with using IP addresses Instructor Note The process of packet transfer depends on IP addresses. This target indicator emphasizes the user-unfriendliness of IP address (which will only get worse when IP addresses are extended, as in IP v. 6). This is why DNS names are necessary. The details of DNS are explored. Students will probably be familiar with the more common domain names. Emphasize the importance of these both for ease of use by humans and for imposing some hierarchical structure on Internet naming. You may want to have the students do some browsing in different domains to explore this topic in a practical context. 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 Web Links DNS: The Domain Name System 15.2 Domain Name System 15.2.2 The domain name server Instructor Note The purpose of this target indicator is for the student to appreciate the sequence of actions involved in a simple DNS lookup. Even within Layer 7, networks involve complex sequences of actions. The domain name server (DNS) is a device on a network that manages domain names and responds to requests from clients to translate a domain name into the associated IP address. The DNS system is set up in a hierarchy that creates different levels of DNS servers. If a local DNS is able to translate a domain name into its associated IP address, it does so, and returns the result to the client. If it cannot translate the address, it passes the request up to the next higher-level DNS on the system, which then tries to translate the address. If the DNS at this level is able to translate the domain name into an associated IP address, it does so, and returns the result to the client. If not, it sends the request to the next higher level. This process repeats itself until the domain name has been translated, or the top-level DNS has been reached. If the domain name cannot be found on the top level DNS, it is considered to be an error and the corresponding error message is returned. Any type of application that uses domain names to represent IP addresses, uses the DNS to translate that name into its corresponding IP address. - Web Links DNS: The Domain Name System
