0 TCP IP STEP 3

Internet host names are an easy way to remember a long decimal IP address. A host name is typically the name of a device that has a specific IP address. A host name and a domain name make up a fully qualified domain name. For example, www.ntshop.net is a fully qualified domain name, where www is the host name and ntshop.net is the domain name. Hosts on a network use names the same way people use names. We all have a Social Security number, and we can certainly remember that number if need be, but imagine if all of your friends and associates had to be called by their Social Security numbers instead of regular names! We use words as names instead because they are easier to remember. Likewise, it’s easier to remember www.ntshop.net than it is to remember 207.91.166.2.

Domain Name Service
Domain Name Service, or DNS, cross-references specific information to translate host names and domain names to IP addresses and vice versa using a lookup table that the network administrator defines and configures. For example, when you type www.ntshop.net in your Web browser, the DNS server returns 206.91.166.2 as an answer, allowing the Web browser to locate the server and load the home page.

Although you can run a TCP/IP network without it, DNS is considered an essential part of any TCP/IP network because it makes the task of remembering addresses much easier. Although humans think best in words, computers work best with numbers. DNS lets the human remember the name while DNS remembers the corresponding numbers, much like a phone book.

DNS tables compile records that are defined in a particular structure. Depending on the record type, the structure can be composed of four or five parts, but for the purposes of this book, we’ll focus on the three important parts: the host name, the record type, and the host address pointer.

DNS tables can store several different types of records. The most common record types are address records, mail exchange records, and alias records. Let’s look at address records, commonly eferred to as A records. A records correlate host names to IP addresses. An address record entry for a host named "mail" in the "ntshop.net" domain is shown below.
mail.ntshop.net. IN A 207.91.166.2
The next record type we’ll look at is the mail exchange record. Mail exchange records point to the mail servers for a given domain name and describe where mail for that domain should be sent. It is possible to specify several mail servers in order of preference for a given domain, thereby increasing the probability that mail will arrive at its destination if one of the servers is down or not responding.