IP Addresses

• The real address of a computer connected to the Internet is called the IP Address. IP stands for Internet Protocol. The IP address is a 32-bit binary number. To make an IP address easier to view, it is broken up into four sections and each number is converted into decimal notation. The four numbers are then separated by dots, resulting in something that looks like “123.34.33.101.”
  
  

Domain Name Servers

• Domain names are easier for the public to remember, but every computer in the world is contactable by an IP address, not a domain name. The Domain Name System maps between these two addressing systems. The database containing all the mappings between domain names and IP addresses in the world is distributed among many computers all over the world. Each of these is called a Domain Name Server, and each holds only a part of the total database.
  
  

DNS Resolvers

• The Web browser receives instructions to fetch a Web page from a user in terms of a domain name. It can only get that page by dealing with other computers using IP addresses. The Web browser hands the task of finding the IP address for a domain name to a DNS resolver. The resolver may be on the same network, or may be resident on an Internet service provider's system. The resolver contacts a Domain Name Server, but that server might not hold the required mapping.
  
  

Recursive/Iterative

• A setting of the resolver dictates how it will deal with situations where the first server contacted does not have the required translation. In a recursive query, the contacted DNS server will contact other servers on the resolver's behalf and then return the mapping. In an iterative query, the resolver asks the server to deliver the mapping only if it holds it itself. If the server does not have the required address, it replies with the address of another server the resolver might try. The resolver may use the suggested address or may ignore it and work though a list of DNS servers it has in a database. The repeated queries to many servers by the resolver is an iterative query.