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<HTML><HEAD><TITLE>Chapter 7 -- How the Domain Name System (DNS) Works</TITLE><META></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910"><H1><FONT SIZE=6 COLOR=#FF0000>Chapter 7</FONT></H1><H1><FONT SIZE=6 COLOR=#FF0000>How the Domain Name System (DNS)Works</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#HowIntranetDomainNameSystemDNSServersWork">How Intranet Domain Name System (DNS) Servers Work</A></UL><HR><P>The heart of intranets and the Internet is the Domain Name System(DNS), the way in which computers can contact each other and dothings such as exchange electronic mail, or display Web pages.The Internet Protocol (IP) uses Internet address information andthe DNS to deliver mail and other information from computer tocomputer.<P>You may not realize that every IP address on the Internet is actuallya series of four numbers separated by periods (called dots), suchas 163.52.128.72. It would be impossible for you to remember thesenumeric addresses when you wanted to send e-mail or visit a site.Also, because sometimes numeric IP addresses change, you wouldnever be able to know every time those numeric addresses change.The DNS solves these problems.<P>The DNS creates a hierarchy of domains or groups of computersand it establishes a domain name (also known as an Internet address)for each computer on an intranet or the Internet, using easilyrecognizable letters and words instead of numbers. Major domainsalso have the responsibility for maintaining lists and addressesof the domains that are underneath them. That next level of domainsis responsible for the following level down and so on.<P>An Internet address is made up of two major parts separated byan @ (pronounced at) sign. The first part of the address-to theleft of the @ sign-is the user name, which usually refers to theperson who holds the Internet account, and is often that person'slogin name. The second part of the address, to the right of the@ sign, is the host name or domain name, which identifies thespecific computer where the person has an Internet mail account.Often, the domain name will be the name of the intranet. <P>The rightmost portion of the domain section of the address identifiesthe largest domain and kind of organization where the person hashis or her address. Common domains in the United States are comfor commercial; edu for education; gov for government; mil formilitary; net for network (companies and groups concerned withthe organization of the Internet); and org for organization. Outsidethe United States, only two letters are used to identify the domains,such as au for Australia; ca for Canada; uk for United Kingdom;and fr for France.<P>Typically, an intranet will have its own domain, often the nameof the company that owns it. Mail to be delivered begins witha request to an intranet nameserver first. If the host receivingthe mail is on the intranet, the nameserver will be able to translatethe Internet address into the numeric IP address, and so the mailcan be delivered. If the host isn't on the intranet, the nameservermay have to contact an Internet nameserver. It does this by contactingan Internet root domain name server, which then tells it whichInternet name server to contact. That Internet nameserver willbe able to translate the Internet address into the numeric IPaddress, and again, the mail will be able to be delivered.<H2><A NAME="HowIntranetDomainNameSystemDNSServersWork"><FONT SIZE=5 COLOR=#FF0000>How Intranet Domain Name System (DNS) Servers Work</FONT></A></H2><OL><LI>When a particular URL needs to be contacted, the address withthe URL must be matched to the true IP address. Your Web browserfirst goes to a local DNS server on the company intranet to getthis information. If the IP address is a local one, the DNS serverwill be able to resolve the URL with the IP address. It will sendthe true IP address to your computer.<LI>Your Web browser now has the true IP address of the placeyou're trying to locate. It uses that IP address and contactsthe site. The site sends you the information you've requested.<LI>If the information you have requested isn't on your intranet-andif your local DNS server doesn't have the IP address-the intranetDNS server must get the information from a DNS server on the Internet.The intranet DNS server contacts what's called the <I>root domainserver</I>, which is maintained by a group called the InterNIC.The root domain server tells the intranet server which primarynameserver and secondary nameserver has the information aboutthe requested URL.<LI>The intranet DNS server now contacts the primary nameserver.If the information cannot be found in the primary nameserver,the intranet DNS server contacts the secondary server. One ofthose nameservers will have the proper information. It will thenpass the information back to the intranet DNS server.<LI>The intranet DNS server sends the information back to you.Your Web browser now uses the IP address to contact the propersite.</OL><P>When someone on an intranet wants to contact a location-for example,to visit a Web site-they will type in an address, such as www.metahouse.com.addresses. Instead, it uses IP addresses, which are numericaladdresses, in four 8-bit numbers separated by dots, such as 123.5.56.255.A DNS server, also called a nameserver, matches alphanumeric addressesto their IP addresses, and allows you to contact the proper location.<HR><CENTER><P><A HREF="ch6.htm"><IMG SRC="PC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="#CONTENTS"><IMG SRC="CC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="contents.htm"><IMG SRC="HB.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="ch8.htm"><IMG SRC="NC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><HR WIDTH="100%"></P></CENTER></BODY></HTML>?? 快捷鍵說明
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