Connecting to the internet - typical scenarios
Let's put together the stuff you've learned about networks and the internet to follow what happens when you connect to the internet from a couple of typical locations...
Connecting to the Internet at home
Let’s say you want to look at a webpage about bananas at www.banana.com in America.
You start your computer. Most modern operating systems have built-in support for Internet access, such as the TCP/IP protocol and Dial Up Networking (DUN). You activate your dialup connection to your ISP. DUN gets your 56K V.90 standard modem to dial the PoP (Point of Presence) phone number for your ISP. One of the many modems at your ISP takes the call, and the two modems begin to negotiate the best speed both of them can manage – you can hear them whistle and “shush” as they do it. When the “handshake” is completed, the modem’s speaker is turned off and your ISP assigns your computer a dynamic IP address from the pool of addresses it owns. You are now part of the ISP’s WAN, and the Internet.
You start up your web browser and type in the URL of your friend Steve’s favourite site ‘www.banana.com.’
The URL is looked up in the ISP’s DNS server - but the DNS does not contain that URL! The ISP’s DNS contacts a bigger, higher-level DNS elsewhere in the world and requests the IP address. This DNS has www.banana.com listed, and it returns the right IP address to you. Now it has an IP address, your browser issues an HTTP command to the IP address, requesting the web page from the web server.
The HTTP command is put in an IP packet along with your IP address as the sender and the IP address of www.banana.com as the destination. The browser sends the packet to your ISP (at 33Kbps – the fastest a 56K modem can upload), who uses the IP protocol to read the destination address and pick the best server to toss the packet to. The packet passes through various other servers, each one using the IP protocol to shunt the packet in generally the right direction. Routers bounce it like a pinball from server to server, from satellite to satellite, across submarine cables, through ISDN lines, between microwave dishes - until it arrives at its destination. The web server knows the packet is addressed to him, so he opens it and reads the HTTP request for a file.
Since you didn’t specify a file name to load, the web server that hosts the www.banana.com looks for a file called “index.htm”, and finds it. The server uses the rules in the HTTP protocol to send index.htm. Again, IP routes the packets to their destination, your TCP checking their packet numbers as they arrive. When they have all arrived (at a maximum speed of 56Kbps), TCP recombines them back into ‘index.htm’ and the browser uses the file’s HTML tags to decide how it should be displayed. Your own browser, and your ISP cache the downloaded file for quick retrieval if it is needed again soon.
The web page appears on your screen and you read all the latest news about the banana world – as many sensible people do (unless they prefer melons.)
Connecting to the internet at school
You log in to an Ethernet-networked school computer by entering your login name and password. The NOS encrypts the password and compares it with the encrypted password stored under your login name. If they match, the network may do several things:
- load any personal settings such as desktop settings, menus
- map network drives, such as your home directory
- run login scripts based on groups you belong to
- assign rights to network resources
- use the DHCP server to assign your computer an IP address
When you run your web browser, your request for www.banana.com is sent through your network card, which uses Carrier Sense to wait for a quiet moment on the LAN to transmit its packet. The packet will pass through the CAT5 cable from the NIC to your network point and pass to a switch, which is where all the other computers in the room also join up. A single cable from the switch leads back to another switch in the secure file server room, where it joins many other cables leading from other computer rooms. This switch has a cable that leads to the file server – the heart of the network.
The file server will ask the proxy server if the requested page from banana.com has already been downloaded and cached. If it has been, the file will be delivered instantly from the proxy server instead of having to get it from the Internet. If the file is not cached, it must be downloaded. Your IP packet containing the HTTP request heads from the file server through a router that protects the LAN from the outside world, and towards the school’s ISP, probably along ISDN cables. From then on, the process is the same as dialing up from home.