DNS-domain name system

Domain Name servers[ DNS ]do two things all day long: 

   “  They accept requests from programs to convert domain names into IP addresses. 

   “  They accept requests from other name servers to convert domain names into IP addresses. 

When a request comes in, the domain names server can do one of four things with it: 

   “  It can answer the request with an IP address because it already knows the IP address for the domain.  

   “  It can contact another name server and try to find the IP address for the name requested.It may have to do this multiple times.

   ”  It can say, “I don’t know the IP address for the domain you requested, but here’s the IP address for a name server that knows more than I do.” 

   “  It can return an error message because the requested domain name is invalid or does not exist. 

When you type a URL into your browser, the browser’s first step is to convert the domain name and host name into an IP address so that the browser can go request a Web page from the laptop at that IP address  To do this conversion, the browser has a conversation with a name server.

When you set up your laptop on the Internet, you (or the software that you installed to connect to your ISP) had to tell your laptop what name server it should use for converting domain names to IP addresses. On some systems, the DNS is dynamically fed to the laptop when you connect to the ISP, and on other laptops it is hard-wired. If you are working on a Windows 95/98/ME laptop, you can view your current name server with the command WINIPCFG.EXE (IPCONFIG for Windows 2000/XP). On a UNIX laptop, type nslookup in Run  with your laptop name. Any program on your laptop that needs to talk to a name server to resolve a domain name knows what name server to talk to because it can get the IP address of your laptop’s name server from the operating system

The browser therefore contacts its name server and says, “I need for you to convert a domain name to an IP address for me.” For example, if you type “www.ezinedomain.com” into your browser, the browser needs to convert that URL into an IP address. The browser will hand “www.ezinedomain.com” to its default name server and ask it to convert it. 

The name server may already know the IP address for www.ezinedomain.com. That would be the case if another request to resolve www.ezinedomain.com came in recently (name servers cache IP addresses to speed things up). In that case, the name server can return the IP address immediately. Let’s assume, however, that the name server has to start from scratch. 

A name server would start its search for an IP address by contacting one of the root name servers. The root servers know the IP address for all of the name servers that handle the top-level domains. Your name server would ask the root for www.ezinedomain.com, and the root would say (assuming no caching), “I don’t know the IP address for www.ezinedomain.com, but here’s the IP address for the COM name server.” Obviously, these root servers are vital to this whole process, so:

   “  There are many of them scattered all over the planet. 

   “  Every name server has a list of all of the known root servers. It contacts the first root  server in the list, and if that doesn’t work it contacts the next one in the list, and so on. 

The root server knows the IP addresses of the name servers handling the several hundred top-level domains. It returns to your domain name server the IP address for a name server for the COM domain. Your domain name server then sends a query to the COM name server asking it if it knows the IP address for www.ezinedomain.com. The name server for the COM domain knows the IPaddresses for the name servers handling the EZINEDOMAIN.COM domain, so it returns those. Your name server then contacts the name server for EZINEDOMAIN.COM and asks if it knows the IP address for www.ezinedomain.com. It does, so it returns the IP address to your name server, which returns it to the browser, which can then contact the server for www.ezinedomain.com to get a Web page. 

One of the keys to making this work is redundancy. There are multiple name servers at every level, so if one fails, there are others to handle the requests. There are, for example, three different laptops running name servers for EZINEDOMAIN.COM requests. All three would have to fail for there to be a problem. 

The other key is caching. Once a name server resolves a request, it caches all of the IP addresses it receives. Once it has made a request to a root server for any COM domain, it knows the IP address for a name server handling the COM domain, so it doesn’t have to bug the root servers again for that information. Name servers can do this for every request, and this caching helps to keep things from bogging down. 

Name servers do not cache forever, though. The caching has a component, called the Time To Live (TTL), that controls how long a server will cache a piece of information. When the server receives an IP address, it receives the TTL with it. The name server will cache the IP address for that period of time (ranging from minutes to days) and then discard it. The TTL allows changes in name servers to propagate. Not all name servers respect the TTL they receive, however. When Ezinedomain moved its laptops over to new servers, it took three weeks for the transition to propagate throughout the Web.

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