How Does The Internet Work

The internet & web design PhD. program information & libraries dep. By Dr. talal azzuhairi 2009-2010

First lecture How does the internet work

How does the internet work 

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Internet Addresses: Because the Internet is a global network of computers each computer connected to the Internet must have a unique address. Internet addresses are in the form nnn.nnn.nnn.nnn where nnn must be a number from 0 - 255. This address is known as an IP address. (IP stands for Internet Protocol)

Internet Addresses The picture above Show two computers connected to the Internet; your computer with IP address 1.2.3.4 and another computer with IP address 5.6.7.8. The Internet is represented as an abstract object in-between.  If you connect to the Internet through an Internet Service Provider (ISP), you are usually assigned a temporary IP address for the duration of your dial-in session. If you connect to the Internet from a local area network (LAN) your computer might have a permanent IP address 

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Protocol Stacks and Packets So your computer is connected to the Internet and has a unique address. How does it 'talk' to other computers connected to the Internet?  An example should serve here: Let's say your IP address is 1.2.3.4 and you want to send a message to the computer 5.6.7.8. The message you want to send is "Hello computer 5.6.7.8!".  The message must be transmitted over whatever kind of wire connects your computer to the Internet 

Protocol Stack 

Let's say you've dialed into your ISP from home and the message must be transmitted over the phone line. Therefore the message must be translated from alphabetic text into electronic signals, transmitted over the Internet, then translated back into alphabetic text.

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How is this accomplished Through the use of a protocol stack. Every computer needs one to communicate on the Internet and it is usually built into the computer's operating system (i.e. Windows, Unix, etc.)

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TCP/IP

Transmission Control Protocol/Internet Protocol

The protocol stack used on the Internet is referred to as the TCP/IP protocol stack. because of the two major communication protocols used.  The TCP/IP stack looks like this: 

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Protocol Layer 

If we were to follow the path that the message "Hello computer 5.6.7.8!" took from our computer to the computer with IP address 5.6.7.8, it would happen something like this:

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Hallo

Send Steps  

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The message would start at the top of the protocol stack on your computer and work it's way downward. If the message to be sent is long, each stack layer that the message passes through may break the message up into smaller pieces of data. This is because data sent over the Internet (and most computer networks) are sent in manageable pieces. On the Internet, these pieces of data are known as packets. The packets would go through the Application Layer and continue to the TCP layer. Each packet is assigned a port number. Ports will be explained later, but suffice to say that many programs may be using the TCP/IP stack and sending messages. We need to know which program on the destination computer needs to receive the message because it will be listening on a specific port. After going through the TCP layer, the packets proceed to the IP layer. This is where each packet receives it's destination address, 5.6.7.8. Now that our message packets have a port number and an IP address, they are ready to be sent over the Internet. The hardware layer takes care of turning our packets containing the alphabetic text of our message into electronic signals and transmitting them over the phone line.

Receive steps 

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On the other end of the phone line your ISP has a direct connection to the Internet. The ISPs router examines the destination address in each packet and determines where to send it. Often, the packet's next stop is another router. More on routers and Internet infrastructure later. Eventually, the packets reach computer 5.6.7.8. Here, the packets start at the bottom of the destination computer's TCP/IP stack and work upwards. As the packets go upwards through the stack, all routing data that the sending computer's stack added (such as IP address and port number) is stripped from the packets. When the data reaches the top of the stack, the packets have been re-assembled into their original form, "Hello computer 5.6.7.8!"

Networking Infrastructure 

Now you know how packets travel from one computer to another over the Internet. But what's in-between? What actually makes up the Internet? Let's look at another diagram:

Definitions Modem: Is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.  Modem pool: Device contain collections of modems  ISP port server: Internet Service Provider, a company that provides access to the Internet. For a monthly fee, the service provider gives you a software package, username, password and access phone number. Equipped with a modem, you can then log on to the Internet and browse the World Wide Web and USENET, and send and receive e-mail. 

Router: A device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISPs network. Routers are located at gateways, the places where two or more networks connect.  CSU/DSU: Channel Service Unit/Data Service Unit) is a hardware device about the size of an external modem that converts a digital data frame from the communications technology used on a local area network (LAN)  ISP backbone: A collection of wires through which data is transmitted from one part of a computer to another 

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Computer Network Hierarchy

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Every computer that is connected to the Internet is part of a network, even the one in your home. For example, you may use a modem and dial a local number to connect to an Internet Service Provider (ISP). At work, you may be part of a local area network (LAN), but you most likely still connect to the Internet using an ISP that your company has contracted with. When you connect to your ISP, you become part of their network. The ISP may then connect to a larger network and become part of their network. The Internet is simply a network of networks. Most large communications companies have their own dedicated backbones connecting various regions. In each region, the company has a Point of Presence (POP). The POP is a place for local users to access the company's network, often through a local phone number or dedicated line. The amazing thing here is that there is no overall controlling network. Instead, there are several high-level networks connecting to each other through Network Access Points or NAPs.

The Function of an Internet Router All of these networks rely on NAPs, backbones and routers to talk to each other. What is incredible about this  Process is that a message can leave one computer and travel halfway across the world through several different networks and arrive at another computer in a fraction of a second! 

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The routers determine where to send information from one computer to another. Routers are specialized computers that send your messages and those of every other Internet user speeding to their destinations along thousands of pathways.  A router has two separate, but related, jobs: 1. It ensures that information doesn't go where it's not needed. 2. It makes sure that information does make it to the intended destination. 

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Routers It makes sure that information does make it to the intended destination 

So how do packets find their way across the Internet? Does every computer connected to the Internet know where the other computers are? Do packets simply get 'broadcast' to every computer on the Internet? The answer to both the preceding questions is 'no‘. computer do not knows where any of the other computers are, and packets do not get sent to every computer. The information used to get packets to their destinations are contained in routing tables kept by each router connected to the Internet.

Routers are packet switches A router is usually connected between networks to route packets between them.  Each router knows about it's sub-networks and which IP addresses they use.  The router usually doesn't know what IP addresses are 'above' it. Examine Diagram 5 below. The black boxes connecting the backbones are routers.  The larger NSP backbones at the top are connected at a NAP.  Under them are several sub-networks, and under them, more sub-networks. At the bottom are two local area networks with computers attached. 

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When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks it's routing table. If the network containing the IP address is found, the packet is sent to that network. If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone. The routers connected to the NSP backbones hold the largest routing tables and here the packet will be routed to the correct backbone, where it will begin its journey 'downward' through smaller and smaller networks until it finds it's destination.

Domain Names and Address Resolution 

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But what if you don't know the IP address of the computer you want to connect to? What if the you need to access a web server referred to as www.anothercomputer.com? How does your web browser know where on the Internet this computer lives? The answer to all these questions is the Domain Name Service or DNS. The DNS is a distributed database which keeps track of computer's names and their corresponding IP addresses on the Internet. Many computers connected to the Internet host part of the DNS database and the software that allows others to access it. These computers are known as DNS servers. No DNS server contains the entire database; they only contain a subset of it. If a DNS server does not contain the domain name requested by another computer, the DNS server re-directs the requesting computer to another DNS server.

Firewall 

A firewall is simply a program or hardware device that filters the information coming through the Internet connection into your private network or computer system. If an incoming packet of information is flagged by the filters, it is not allowed through.

Firewall Methods  

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Firewalls use one or more of three methods to control traffic flowing in and out of the network: Packet filtering - Packets (small chunks of data) are analyzed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded. Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa. Tasteful inspection - A newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.

Firewall Configuration 

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Firewalls are customizable. This means that you can add or remove filters based on several conditions. Some of these are: IP addresses - Each machine on the Internet is assigned a unique address called an IP address. IP addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this: 216.27.61.137. For example, if a certain IP address outside the company is reading too many files from a server, the firewall can block all traffic to or from that IP address. Domain names - Because it is hard to remember the string of numbers that make up an IP address, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, it is easier for most of us to remember www.howstuffworks.com than it is to remember 216.27.61.137. A company might block all access to certain domain names, or allow access only to

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Protocols - The protocol is the pre-defined way that someone who wants to use a service talks with that service. The "someone" could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have their conversation. The http in the Web's protocol. Some common protocols that you can set firewall filters for include:         

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IP

(Internet Protocol) - the main delivery system for information over the Internet TCP (Transmission Control Protocol) - used to break apart and rebuild information that travels over the Internet HTTP (Hyper Text Transfer Protocol) - used for Web pages FTP (File Transfer Protocol) - used to download and upload files UDP (User Datagram Protocol) - used for information that requires no response, such as streaming audio and video ICMP (Internet Control Message Protocol) - used by a router to exchange the information with other routers SMTP (Simple Mail Transport Protocol) - used to send textbased information (e-mail) SNMP (Simple Network Management Protocol) - used to collect system information from a remote computer Telnet - used to perform commands on a remote computer

Proxy Servers A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server. Proxy servers have two main purposes:  Improve Performance: Proxy servers can dramatically improve performance for groups of users. This is because it saves the results of all requests for a certain amount of time. Consider the case where both user X and user Y access the World Wide Web through a proxy server. First user X requests a certain Web page, which we'll call Page 1. Sometime later, user Y requests the same page. Instead of forwarding the request to the Web server where Page 1 resides, which can be a time-consuming operation, the proxy server simply returns the Page 1 that it already fetched for user X. Since the proxy server is often on the same network as the user, this is a much faster operation. Real proxy servers support hundreds or thousands of users. The major online services such as America Online, MSN and Yahoo, for example, employ an array of proxy servers.  Filter Requests: Proxy servers can also be used to filter requests. For example, a company might use a proxy server to prevent its employees from accessing a specific set of Web sites. 

Web servers 

A web site is a collection of web pages. And web pages are digital files, typically written using Hypertext Markup Language (HTML). For a web site to be available to everyone in the world at all times, it need to be stored or "hosted" on a computer that is connected to the internet. Such a computer is known as a web Server You can potentially host a web site on your home computer but this involves a lot of work and constant monitoring. It is easier to "buy" web hosting from a company because there are thousands that offer this service.

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