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Dated: Jul. 08, 2012
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Networking In General
Overview of Domain Name System (DNS) Servers
Domain Name System or DNS servers are dedicated computers that have network operating systems installed on them and are responsible for resolving hostnames (computer names) to their IP addresses and vice versa. DNS servers are used worldwide and are helpful while accessing the Internet and visiting websites. Every Internet Service Provider or ISP has its own DNS server which receives name resolution requests from the subscribers of that ISP and resolves the requested host names to their corresponding IP addresses.
DNS servers are always important because all the websites (that reside in their respective servers) that are present on the Internet can be contacted only through their corresponding IP addresses. However, the matter of fact is that it is practically impossible for human beings to memorize IP addresses of all the websites available around the globe. It is rather easier for people to learn the names of the websites instead of their IP addresses.
When users type the names of the websites in the address bars of the web browsers (e.g. Internet Explorer), there should be some method to resolve the names to their corresponding IP addresses in order to establish communication between the requesting computers and servers on which the websites reside. This name resolution is done with the help of DNS servers.
Environments Where DNS Servers Are Installed, Configured and Used
DNS is a server roll that administrators install the network operating systems. As mentioned above, every ISP has its own DNS server. Also, there are some root DNS servers that are available on the Internet to which DNS servers of the ISPs forward their queries in case they fail to resolve them on their own. The entire list of root DNS servers can be seen in the Root Hints tab of the DNS servers’ properties box. In all there are 13 root hint servers available worldwide.
Apart from using DNS servers to communicate with the computers on the Internet, they are also installed in local area networks where there are several computers connected to each other. Unlike small network setups, where memorizing the IP addresses of the computers is easier because of limited numbers of computers, in large LAN environments it becomes essential to have DNS servers to resolve hostnames to their corresponding IP addresses. This is because it is impractical to memorize the IP addresses of several computers. This becomes even more challenging if the IP addresses are assigned dynamically through DHCP servers and are likely to change every now and then. Since hostnames are more user-friendly, it becomes easier for the users to memorize them and access the remote computers using their host names. In these cases DNS servers help them by resolving the requested hostnames to their IP addresses.
Types of DNS Queries
DNS queries are divided into two categories. They are:
Iterative Query – In iterative DNS query, the DNS server communicates with all the DNS servers to get the references of the next hop in order to reach the destination host. For example if DNS server A wants to communicate with host Z, A would send an iterative query to DNS server B which would revert back with the reference of DNS server C. A would then send a query to C that would again revert back with the reference of DNS server D and so on. This iteration carries on his A gets the IP address of Z. In this type of query, it is A that moves to and fro and takes all the pain to resolve the hostname to its corresponding IP address. In most cases, iterative query is sent from one DNS server to the other DNS server.
Recursive Query – This type of query is mostly initiated by DNS client computers. When a DNS client computer sends a query to its DNS server for name resolution, the DNS server initiates iterative query to resolve the name of the request host to its IP address. Once the hostname is resolved, the DNS server forwards the final result (IP address of the requested host) to the DNS client computer. Since the DNS client computer sends a query the DNS server and receives the final answer from it, the query initiated by the DNS client computer is called recursive query.
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