Computer Networks

What Is a Network?

A computer network is when two or more computers are connected so they can share data. Usually, networks consist of a large number of computers, though two is the smallest number.

A little history: before the 1960s, computers were relatively rare. They were usually very large, expensive machines which worked alone. Eventually, the U.S. military had many computers all over the United States, and it wanted to connect them together so they could communicate with each other. Some say that the military had another motive: to create a communications network which could survive a nuclear attack. However, this is not true; while such ideas were studied, it was never the primary intent in creating the network--the primary intent was simply to connect the computers together.

A scene from the movie "Dave"


One problem that had to be solved was compatibilityThe ability of different machines and/or programs to work together and communiate smoothly. Most computers were custom-built, and had their own ways of using data, most of which could not work well with other computers. In order to make sure that all computers could communicate smoothly, protocolsA "protocol" is an agreement to do the same thing in the same way. were created.

"Protocol" is a word that existed long before computers. Protocols are used whenever people communicate or act together, to make sure everybody does things the same way. For example, each country may have its own protocols for greeting. In Japan, people bow to each other. In America, people shake hands. Each method has its own rules: in Japan, you bow more deeply to people "above" you; in America, the number of shakes, strength of the grip, and even the way you hold the other person's hand are all details of the protocol.

If people do not have the same protocols, there can be trouble. People will not know how to act, and things can get confused. The scene (at right) from the movie "Dave" might be one example; you may have even experienced yourself the trouble meeting a person with a different protocol, where one of you tries to shake hands at the exact same time that the other person decides to bow.


Computers have similar problems. In order to work in a network, they need protocols. Remember, a "protocol" is an agreement (or rule) for everyone to act the same way or use the same language. If all computers follow the same protocols, they can communicate more smoothly. The main protocol used in networking is TCP/IPTransmission Control Protocol / Internet Protocol, also known as the Internet Protocol Suite. This is a group of protocols which allows for basic connection and data transfer, in which data is divided into "packets," given port and address labels as they pass through different "layers," and then sent to other computers, which "unwrap" the packets and put them together again. Other protocols may tell computers what format the data must be so that each computer can read it correctly.

There are many protocols used widely on the Internet. You know the protocol for the Web, don't you? When you type a web page address, the first letters you might type are HTTP, which stands for Hyper Text Transfer Protocol, meaning a protocol for exchanging "hypertext," which is web pages using links. If you send email, then you are probably using POP, or Post Office Protocol (POP is for receiving mail, SMTPSimple Mail Transfer Protocol is for sending it). There are protocols for sending files (FTPFile Transfer Protocol), and so on. Sometimes protocols are private; Skype uses a protocol for its audio and video chat, but keeps it private. Protocols can be included in programs you download to your computer.


Network Connections

Computers are usually connected in a network using one of two ways: cables and wireless. The common cable used for networking is called EthernetIn English, "Ether" describes a space or area. Ethernet cables are so common that they are also called Network cables or LAN cables.

Using Ethernet cables, you could simply connect two computers directly. Every computer has a LAN (Ethernet) port. An Ethernet cable could be connected to both ports, allowing the computer to "talk." However, 2-computer networks are almost never used; even if there are only two computers at first, there is usually a desire to have "openings" for more computers.

However, thers is a problem: if each computer has only one LAN port, then how do you connect more than two computers together? The first two could connect just fine, but the third would have no place to plug in.

In order to make it work, you need a special device called a hub. The hub allows several cables to be connected, and moderates the flow of information.

In networks also connected to the Internet, the cable leading from the Internet gateway is added, as seen above (often plugged into a special port on the hub).


Then there are wireless options for networks. The most common type is called WiFi. A WiFi base station or access point will send and receive data from computers by radio signals. Usually, WiFi stations are also connected to the Internet via an Ethernet cable.

Many people also use cell phone networks, with "3G" the most common method used today. You can get this signal using a cell phone or other mobile device. Although 3G is intended primarily for cell phones, computers can access a 3G Internet connection using a special PC Card which plugs into your computer. These cards usually have a little antenna on them.

A new form of wireless networking is WiMAX, a higher-speed version of wireless networking similar to 3G. WiMAX, however, is intended primarily for computer networks.


What Can Connect to a Network?

Computers are not the only devices that connect to computer networks. Any number of peripherals and mobile devices can also join. Here is a list of a few, perhaps the most common network members:

  • Desktop Computer
  • Laptop (Notebook) Computer
  • Tablet Computer (e.g., iPad; WiFi only)
  • Network Printer
  • Network Hard Drive
  • Cell Phone or PDA (WiFi only)
  • Digital Camera with WiFi card

Types of Networks

There are two basic types of computer networks: the LAN (Local Area Network), and the WAN (Wide Area Network). In the simplest for, a LAN is a network of computers limited within one building or campus; a WAN is when computers are connected between widely separated locations.

Other special types of networks are sometimes referred to:

  • PAN: Personal Area Network, used to connect different personal devices together
  • HAN: Home Area Network, for inside a residence
  • CAN: Campus Area Network, used by colleges or other groups of buildings
  • MAN: Metropolitan Area Network, used for individual cities

The PAN, HAN, and CAN are often just referred to as LANs, and a MAN falls under the WAN category.


Network Addresses

Each computer on a network has at least one address. This is called an IP Address"IP" stands for "Internet Protocol".

Traditionally, IP addresses are in the form of a 32-bit number, divided into 8-bit parts.

Because each 8-bit number can only have 256 possibilities (0 ~ 255), no number is higher than 255.


There are two kinds of IP Addresses: Internal and External.

An Internal IP Address is the address of a computer in a LAN. This is a private address, similar to a room number in an apartment building. Internal IP addresses must use special numbers reserved for LANs. The special numbers are ("x" means any number between 0 and 255):

  • 192.168.x.x
  • 172.16.x.x ~ 172.31.x.x
  • 10.x.x.x

If you see a computer address using these patters, for example or, they must be internal IP addresses.

An External IP Address is the address a computer has on the Internet. This is a different address; if you think of the internal address as an apartment number, then the external address would be the address of the apartment building.

It is important to understand that while the External address is public (anyone can see it), your Internal address is private, and only known to people with direct access to the LAN. Since it is possible to have thousands of computers in a LAN, knowing the External address does not identify the user.

For home networks created using an ISP, however, the Internal address is known to the ISP, and records of computer use are kept. If, for example, you commit a crime using your Internet access, the police will be able to see your external address--but they can then go to the ISP and demand to see the internal address records as well. In short, the "privacy" is not complete.

This is an issue in music, movie, and software piracy; companies which sell these goods might "see" people downloading illegal versions of their products; however, they can only see the public external IP addresses. Unlike the police, they do not have the right to demand ISPs give them the internal records as well. Some ISPs, however, will share this information. College campuses very often give this information out. If you live on a college campus, do not assume that you have any privacy!