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This one is called traceroute and it shows the path your packets are taking to a given Internet destination. Like ping, you must use traceroute from a command prompt. In Windows, use tracert www. From a Unix prompt, type traceroute www. Like ping, you may also enter IP addresses instead of domain names. Traceroute will print out a list of all the routers, computers, and any other Internet entities that your packets must travel through to get to their destination.

If you use traceroute, you'll notice that your packets must travel through many things to get to their destination.

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Most have long names such as sjc2-core1-h These are Internet routers that decide where to send your packets. Several routers are shown in Diagram 3, but only a few. Diagram 3 is meant to show a simple network structure. The Internet is much more complex. Internet Infrastructure The Internet backbone is made up of many large networks which interconnect with each other.


These networks peer with each other to exchange packet traffic. NAPs were the original Internet interconnect points.

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Below is a picture showing this hierarchical infrastructure. Diagram 4 This is not a true representation of an actual piece of the Internet. None of the physical network components are shown in Diagram 4 as they are in Diagram 3. This is because a single NSP's backbone infrastructure is a complex drawing by itself.


Most NSPs publish maps of their network infrastructure on their web sites and can be found easily. To draw an actual map of the Internet would be nearly impossible due to it's size, complexity, and ever changing structure. 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 preceeding questions is 'no'. No computer 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.

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Under them are several sub-networks, and under them, more sub-networks. At the bottom are two local area networks with computers attached. Diagram 5 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. What if the you need to access a web server referred to as www. How does your web browser know where on the Internet this computer lives? 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. The computer requesting a name resolution will be re-directed 'up' the hierarchy until a DNS server is found that can resolve the domain name in the request.

Figure 6 illustrates a portion of the hierarchy. At the top of the tree are the domain roots. Some of the older, more common domains are seen near the top. What is not shown are the multitude of DNS servers around the world which form the rest of the hierarchy.

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When an Internet connection is setup e. This way, any Internet applications that need domain name resolution will be able to function correctly. For example, when you enter a web address into your web browser, the browser first connects to your primary DNS server. After obtaining the IP address for the domain name you entered, the browser then connects to the target computer and requests the web page you wanted. Right click on your Internet connection and click Properties. If you have a permanent connection to the Internet: Right click on Network Neighborhood and click Properties.

Select the DNS Configuration tab at the top. You will probably have to restart Windows as well. Now enter an address into your web browser. The browser won't be able to resolve the domain name and you will probably get a nasty dialog box explaining that a DNS server couldn't be found. However, if you enter the corresponding IP address instead of the domain name, the browser will be able to retrieve the desired web page.

Other Microsoft operating systems are similar.

HTTP is a connectionless text based protocol. Clients web browsers send requests to web servers for web elements such as web pages and images. After the request is serviced by a server, the connection between client and server across the Internet is disconnected. A new connection must be made for each request.

Most protocols are connection oriented. This means that the two computers communicating with each other keep the connection open over the Internet. HTTP does not however. Before an HTTP request can be made by a client, a new connection must be made to the server.

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When you type a URL into a web browser, this is what happens: If the URL contains a domain name, the browser first connects to a domain name server and retrieves the corresponding IP address for the web server. The web browser connects to the web server and sends an HTTP request via the protocol stack for the desired web page. The web server receives the request and checks for the desired page.

If the page exists, the web server sends it.

If the server cannot find the requested page, it will send an HTTP error message. The web browser receives the page back and the connection is closed. The browser then parses through the page and looks for other page elements it needs to complete the web page. These usually include images, applets, etc. For each element needed, the browser makes additional connections and HTTP requests to the server for each element.