A Beginner's Guide to the Internet

Contents

Introduction

The Internet has become an integral part of daily life. Unfortunately, while many people have come to rely on it, most have little or no understanding of how the Internet works. This can pose serious problems, particularly as issues around information security become more common and more serious. This article will set out to explain to readers who have little or no technical background how the Internet works . It is hoped that this will provide those readers with a good fundamental knowledge of the principles and terminology of the Internet, so that they can begin to protect themselves against the variety of security threats that exist on the Net.

The Internet as Information Superhighway

Almost everyone has heard the Internet referred to as the Information Superhighway. The metaphor is appropriate - similar to the system of highways and roadways that allows us to drive almost anywhere in the nation, the Internet is an interconnected network of networks. Like a superhighway, the Internet transports vast amounts of information traffic from point to point at very high speeds along telephone lines and fibre-optic cable, and by satellites and microwave links. It allows users to hook up with friends and family and to conduct business around the world.

The Information Superhighway

Web Browser

Internet travelers navigate their journey by means of a web browser. The browser can be thought of as the window onto the Internet, it is a software application used to locate and display Web pages. The two most popular browsers are Mozilla Firefox and Microsoft Internet Explorer. Both of these are called graphical browsers, which means that they can display graphics as well as text. In addition to graphics and text, most modern browsers can also present multimedia information, including sound, video, and moving animation.

Local Area Networks

Your computer may be connected to another computer within your home or your office. If that is the case, then you are part of a LAN, a local area network. A LAN is a series of computers that is connected on a small network. You can think of a LAN as a neighborhood of computers that are connected and are generally situated within close physical proximity. Most LANs are contained within a floor of a building, a building or a group of buildings. LANs link computers together and provide shared access to printers, file servers, and other services. The LAN server then connects all those devices to other networks, such as the Internet. A network is just a way of connecting computers. The internet, is a very big network.

A Local Area Network

In actual fact the picture above is slightly misleading. Internet security is now a high priority. Most networks (LANs), connect to the internet through the server. This allows them to make use of a piece of software called a Firewall.

Firewalls

Whether your connection to the internet is through a server on the LAN you are part of, or even if you have a direct connection to the internet, your connection certainly ought to be through a firewall. A firewall is a program that determines what sort of traffic can come through the connection. This can help stop hacker accessing your machine (or even knowing that it exists as they scan the internet). Unfortunately, they can also be used to stop those on networks from having access to the internet. Here is part of what the Linux Dictionary has to say about firewalls.

A firewall is used on some networks to provide added security by blocking access to certain services in the private network from the rest of the internet, in the same way that a firewall in a building keeps fire from spreading, an internet firewall keeps hackers from spreading.

Internet Service Provider

Whether you are part of a LAN or you are trying to connect to the Internet as a single user, you will likely gain access to the Internet via an Internet Service Provider, or ISP. An ISP is essentially an off-ramp onto the Internet. ISPs are companies that allow you to connect to their computers, which in turn are connected to the Internet. ISPs are usually local companies that provide individuals and small companies with all the requirements to allow user to connect to the Internet. This may include software, domain names, user names and connectivity.

Routers

A router is a computer that sits on a network. Its sole purpose is to direct messages to their final destination. Routers are important because messages do not always travel the same path to their final destination. As the Internet has gained in popularity the various channels of the Internet have become increasingly busy. Often the channels that link computers become overloaded with traffic. Like a policeman on a very busy road, the router may detect that there is a backlog of traffic on one of the routes to a message's destination, in which case it will simply send the data along a different, more convenient route.

The Internet Backbone

As you travel along the Internet, the router will direct you onto a network that is the equivalent of an Interstate highway - the Internet backbone. The Internet backbone is the bulwark of the Internet, spanning the globe and moving large amounts of information across vast distances at high speeds. The Internet Backbone consists of three components. The first component is made up of phone lines and cables that make up the channels along which information travels. The second component, the Network Service Providers (NSPs) provide high-speed Internet access and services for Internet Service Providers, who then make these services available to individual Internet subscribers. Finally, Network Access Points (NAPs) allow messages to 'hop' or transfer from one network to another, thereby providing the connection between different networks. The three components work together to facilitate the high-speed movement of information around the world.

How the Internet Works

By now, it is hoped that you have a decent idea of how the Internet is constructed. However, in addition to understanding the structure of the Net, we must also understand how computers communicate with each other in order to send, receive and understand messages.

Protocols

On any road or highway, there must be a system of rules and procedures that allows traffic to navigate to its chosen destination. On the Internet, this system of order is known as protocols. The term protocol describes a set of rules for encoding and decoding data so that messages may be exchanged between computers so that each computer can fully understand their meaning. Simply put, protocols allow computers and networks to speak to each other in order to exchange data.

TCP/IP

The set of protocols that allows the Internet to operate is known as TCP/IP. TCP/IP, which stands for Transmission Control Protocol/ Internet Protocol, allows different computer platforms (such as Mac or PC) utilizing different operating systems (such as Windows, MacOS or Unix) or different software applications to be able to communicate. Although TCP and IP are two distinct protocols, each of which serves a specific communicational purpose, the term TCP/IP is used to refer to a set of protocols that include many different protocols. For instance, a couple of protocols that you may have heard of are HTTP - Hypertext Transfer Protocol, the protocol used to serve web pages - and FTP - File Transfer Protocol, the protocol used on Internet file servers. For the sake of simplicity, we will stick to discussing only TCP and IP in this article.

Packets

In order to understand how TCP and IP work together to create meaning, it is necessary to understand how messages travel from one computer to another. Messages - such as e-mail messages or web pages - that are sent along the Internet are broken down into smaller packets of information. This is done because there is limited room for message transmission along the Internet. Keeping the messages to a standardized parcel size of fewer than 1500 characters ensures that traffic will continue to move smoothly.

Once the message is broken down into packets, each one is transmitted to the destination computer individually. Packets that make up the same message are often sent along different routes to its destination by the router depending on the amount of traffic on any given channel at the time of transmission. Upon arrival at the destination computer, the packets are recombined into the original message.

Travelling Packets

Packet Headers

Each packet is given a header that contains important information such as the address of the destination computer, the address of the source computer, the amount of data in the packet and how the packets must be recombined to form the original message. The header also contains information known as a checksum. The checksum is a numerical value based on the amount of information contained in the message.

Packet Headers

Role of TCP

So what role does TCP/IP play in this process? TCP (Transmission Control Protocol) is responsible for breaking the messages down into packets. When the destination computer receives the packets it uses TCP to recombine the packets into the original message. By comparing the amount of information indicated in the checksum with the amount of actual information in the message, the destination computer can check to ensure that no errors have occurred in the transmission of the message.

The Role of IP

What role does IP play in the transmission of information? IP (Internet Protocol) ensures that the information gets to the proper destination computer. How? When you send a message or a request for a web page, you specify a URL (Uniform Resource Locator) or computer name where you want the message to go, such as www.securityfocus.com. The distinct name of the company, such as SecurityFocus.com, which is known as a domain name, is actually a verbal code for a numerical address that signifies the location of the destination computer.

Domain Names

Domain names are used instead of the IP address for the simple reason that they are easier to remember. For instance, the domain name SecurityFocus.com is much easier to remember than 207.126.127.66, which is SecurityFocus.com's IP address. All domain names are stored in a database along with their corresponding IP address. When you type in a domain name, an Internet service called the Domain Name Service translates that name into its corresponding IP address so that the desired website can be found.

IP Addresses

TCP/IP requires that each and every computer on the Internet be identified with a particular name and address. If you are connected to the Internet, your address must be unique amongst all computers on the Internet, or else confusion will arise about where information should be sent. The address that the Internet uses to locate you and direct your messages is called an IP address.

The IP address is a set of numbers separated by dots. Every computer or device that is connected to a network must have a unique IP address so that any and all other computers can locate each other particular computer. For example, even if your computer does not perform any server functions, servers need to know your particular computer's address so that they can send it the information that you are requesting. When your computer wants to talk to another computer, it contacts the other computer using its IP address; at the same time, your computer provides its own IP address to the other computer, so that the other computer knows which computer is waiting for an answer and where to direct its response.

Dynamic and Static IP Addresses

Although we have just discussed IP addresses as a single entity, there are actually two types of IP addresses: dynamic and static. If you use a dial-up modem, your IP address will change each time you connect to the Internet. This is because your Internet service provider is assigned a pool of IP addresses to assign to its clients. When you connect to the Internet, you're assigned a unique IP address that identifies your computer. When you disconnect from the Net, the number is returned to the pool and assigned to another subscriber. This type of IP address is known as dynamic because it frequently changes.

People who use always-on connections do not generally disconnect from the Net, even when they are not using their computer. For this reason, their IP address tends to stay the same for a long period of time. This type of IP address is known as a static address. It should be noted, that there will be times when the Internet connection will be disconnected, such as when the machine is being rebooted. There are also users who prefer to shut down their computers when they are not in use, this will also result in a disconnection from the Internet. When the connection is re-established, the IP address the computer is given will be different. However, since many always-on users maintain connnectivity to the Internet for long periods of time, their addresses are referred to as static addresses.

Dial-up Modems

Until relatively recently, dial-up modems were the most common form of connectivity because all they required was that the user have a phone line, which almost everyone does. However, because they rely on phone lines, which have limited transmission capacity, dial-up modems are very slow. As the Internet started to offer more exciting multi-media applications, such as audio, video, dynamic animation, etc, the limitations of dial-ups became very cumbersome.

Always-on Connections

In recent years more high-speed connections have gained in popularity. These include cable, Digital Subscriber Lines, more commonly know as DSL, and Integrated Services Digital Network, or ISDN. The difference between these high-speed connections and dial-up modems does not lie only in the amount of information that is transportable via the connection. Unlike dial-up connections, these methods of connectivity do not tie up phone lines, or other vital services. As a result users are able to keep them connected at all times, without tying up their phone. The figure below depicts a DSL connection.

A DSL Connection

Security Ramifications of Always-on Connections

The difference between dial-up and always-on connections has important ramifications for the discussions of Internet security that follow in the rest of this focus area. Without certain security measures, a computer that is hooked up to the Internet is like an open door - as long as the connection is maintained, information can flow in and out of the computer almost unchecked. This means that as long as the connection to the Internet is on, people who are prowling the Internet can gain access to computers that are logged on to the Internet.

Furthermore, because always-on connections maintain a static IP address, once a potential attacker identifies an always-on connection, he or she will know where to find it in the future in order to attack the computer. On the other hand, if a hacker identifies a dial-up connection, he or she will not necessarily be able to find that machine in the future because the IP address will almost certainly have changed.

As a result, always-on computers have security concerns that are different, and somewhat more urgent than the concerns of those with dial-up connections. However, these concerns should not discourage users from taking advantage of the benefits that higher-speed always-on connectivity offers. Rather, with some knowledge, effort and on-going vigilance, always-on users can adequately protect themselves without expending a lot of time or money.

Conclusion

It is hoped that the brief overview of the Internet that this article has attempted to provide will give Internet users without a lot of technical background enough of an understanding that they can go out and take the necessary steps to protect themselves and their information on-line.

Relevant Links

Twenty Questions: How the Net Works By Scot Finnie, CNET Services

General description of the TCP/IP protocols By Charles L. Hedrick

General Internet FAQs From PacificBell

CapitalNET Glossary of Internet Terms From Capital.net

The Internet: A Tool for Empowering People in the Information Age From the U.S. Agency for International Development


Written by Stephen Entwisle Nov. 6, 2000. Published online at securityfocus.com. Addition on Firewalls by Fuzzyman, 2005.

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Last edited Sun Oct 01 20:10:46 2006.