Understanding the Basics
Domain Names, IP Addresses, Host Names, Web Sites, URLs, and Whois
Let's start with explaining the basic terms.
Domain: A domain is a logical region of the Internet. Domain names consist of one or several parts separated by periods, for example: "yahoo.com." You can refer to all of the computers that share the right-most portion of a name as being in the same domain, for example: "weather.yahoo.com" and "finance.yahoo.com" are both in the "yahoo.com" domain.
The top position in the domain hierarchy is occupied by Top Level Domains (TLDs). A TLD is the right-most portion of a domain name, and you obviously know many popular TLDs, for example: .COM, or .NET. Seven generic TLDs (.COM, .EDU, .GOV, .INT, .MIL, .NET, and .ORG) were created in 1980s, seven new TLDs were introduced in 2001 and 2002 (.BIZ, .INFO, .NAME, .PRO, .AERO, .COOP, and .MUSEUM), and, of course, there are over two hundred country-specific TLDs that consist of two letters, e.g. .CA for Canada or .DE for Germany (the complete list can be found on the IANA web site).
A second-level domain (SLD) is the next portion of a domain name (we're moving from the right to left here). For example, there are millions of second-level domains in the .COM top-level domain zone, one of them being "yahoo.com." "Finance.yahoo.com" or "www.yahoo.com" would be a third-level domain, etc. Every existing SLD has an owner, a company, an organization, or individual and a Whois tool allows you to find the SLD owner.
IP Address: Every computer connected to the Internet is assigned a unique number known as an Internet Protocol (IP) address. This number identifies each sender or receiver of information that is sent in packets across the Internet. The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 18.104.22.168 or 22.214.171.124 could be valid IP addresses.
Your computer, too, is assigned an IP address; because you can't access a web site or use any other Internet service, such as e-mail, without a valid IP address. Many IP addresses are assigned dynamically from a pool, while others are assigned on a permanent basis. Because IP addresses are usually assigned in country-based blocks, an IP address can often be used to identify the country from which a computer is connecting to the Internet.
Host Name: A host name is only an alias for the IP address. It is assigned to a computer for easy reference by humans. Take, for example, a web server, "www.lookup-ip.com." Its IP address is 126.96.36.199, but this number is not easy to remember. It's not likely that you will learn it by heart and enter it into your browser when you want to visit this web site, but www.lookup-ip.com is much easier to remember. When you enter www.lookup-ip.com into your browser, it quickly contacts the Domain Name System (DNS) server and finds out that www.lookup-ip.com translates into 188.8.131.52. From that point on, your browser uses the obtained numeric IP address to send and receive data to/from this web site. A computer connected to the Internet may or may not have a host name, but it must have an IP address.
Here are a few examples of host names: www.google.com (a web site), pop-server.austin.rr.com (a mail server), cs2416713-236.houston.rr.com (a host name assigned to a broadband Internet user), dialup134.ts521.cwt.esat.net (a host name assigned to a dial-up user).
Web Site and URLs: A web site is a group of web pages stored on a particular web server. A web server is just a computer connected to the Internet that runs the software necessary for serving web pages. Typically, web servers have host names that begin with "www," e.g. "www.google.com," although this is not always the case. A Uniform Resource Locator (URL) is the address of a resource that is retrievable using on of the Internet protocols, usually Hyper Text Transfer Protocol (HTTP). An example of a URL is "http://www.google.com/about.html."
Whois: Whois is an Internet program that allows users to query a database of domains and IP addresses to retrieve information about the owners, administrators, geographic location, etc. As the name suggests, SmartWhois is a smart, feature-rich Whois utility capable of performing such queries. You will learn about what you can do with it in the next chapter.