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All these computers that are connected to each other via the Internet need some way to 'find' each other. There is a large system somewhat like a postal system that assigns each computer a special address.
The addresses are called IP numbers. An IP number has four 'digits', with each digit represented by a number from 0 to 255. The smallest IP number is 0.0.0.0 The largest one is 255.255.255.255 The computer that this document was written on has an IP number of 134.68.140.61 These numbers are used very much like a zip code to help a packet of information find its way to the proper destination.
Numbers are, of course, easy for the computer to deal with, but confusing for human users. Computer scientists developed a system called the Domain Name Server (DNS) system. It was devised to give verbal equivalents to IP numbers. Many computers have domain names in addition to IP addresses. The Domain names are paired with the IP numbers in a large database that is distributed throughout the Internet. So far, you may not have used any IP numbers. Most of the computers you access through a web browser have a domain name. You will only need to know about IP numbers when you encounter a computer which does not have a name assigned.
The DNS system is organized in a tree - like structure based loosely on the organization of the ARPANet. All the computers in the Internet are thought of as belonging to a specific wide domain. Each domain might have a number of subdomains, which might have a number of computers in it. Finally, each computer may have a number of users who own or regularly use the computer.
Part of the DNS tree structure is shown here:
There are two main ways of organizing the highest level domains. The ARPANet defined domains with three character abbreviations based on the type of organization which owns the computers. The most common of these names is summarized in the table below:
| Domain | Type of Organization |
| .edu | Educational institution |
| .com | Commercial or business |
| .gov | Government organization |
| .org | Non-profit organization |
You might also find two character domain abbreviations, like .fr or .au. These are international domains, which represent the country of the computer. .fr stands for France, and .au stands for Australia.
Each domain is broken down into a series of subdomains. Subdomains are generally large organizations. In the .edu domain, for example, indiana is a subdomain representing Indiana University, and iupui is another subdomain representing IUPUI.
Within a subdomain, individual computers are also frequently given names. In one computer lab, all the computers are given names of ships or planets from a popular science fiction series.
A computer might have the following complete DNS address: klingon.cs.iupui.edu
From right to left, we can interpret the address in this way:
edu = educational institution This computer belongs to some type of school or other educational institution.
iupui = Indiana University / Purdue University / Indiana This is the name of a particular university. If we did not recognize IUPUI as a university name, we might be stuck here. It would still be reasonable to assume that IUPUI was the name of a school, though, because of the .edu extension.
cs = Computer Science Again, we might have to just guess that cs means computer science. Organizations might abbreviate departments as part of the internal naming structure they choose. CS is a very common abbreviation in universities for computer science.
klingon = the name of the particular computer. The only reason we know this is because it is at the very left hand part of the complete name.
As you can see, there is a lot of guesswork involved in interpreting a domain name, but the letters are not random. There is usually some kind of organization to the name.
Organizations apply to InterNIC for a particular subdomain name. Once they receive that name, they can create subdomains however they want.
To make things a little easier, many subdomain owners have followed a special convention: Since most transactions are done via the worldwide web, a special subdomain is often designated specifically to hold the main documents that organization wants distributed via the web. That subdomain is usually something like: www.orgname.com where orgname is the name of the organization (or some other catchy word) and .com is replaced by whichever domain type is appropriate.
In the Internet world, you see that computers have names. Individual users also have names. The names are usually given to users when they are given account on a Internet connected computer. Most local area networks require each user to have a name. Most mainframe computers have a similar requirement The user's name is often called a login or userid. Frequently these names are based on the user's actual name, but it is often abbreviated and put only in all lower case letters. A user named "Georgia W. Wilson" might be assigned userids like gwilson or gwwilson. The advantage of this is that if we can combine the userid with the domain name of her system to get a complete address. If Georgia had an account on the klingon computer described above, her complete Internet address would be "gwwilson@klingon.cs.iupui.edu". Other Internet users anywhere in the world could send messages to her by using that address.
Security is an important consideration in the Internet world. The computer has know way of knowing who you are. Most LANs or mainframes require each user to log in, or sign on to the system. To do this, you type in your userid at a prompt. You are then prompted for a password.
passwords are special codes you use to 'lock' your account. Obviously you need to be careful about protecting your password if you are building nuclear power plants or something, but what about the rest of us?
A student recently got some help from her friend on email. She gave him her password to a class account, and he wrote it down. Apparently some of his buddies found the password and logged in using the student's userid and password, and sent a very obscene message to the class. The student nearly lost her account, because she had been sloppy with her password.
Change your password regularly!
Some hints: