Before leaving work Bob logs onto his work computer. He uses the intranet, a part of the internet that is used only for businesses, to look up some information of a colleague. He uses a simple mail transfer protocol, SMTP, to send them an e-mail containing a link on the extranet; it is sent to the staff only e-mail notice board.
Bob then goes home, meanwhile after the e-mail had been sent the transport layer (TCP Layer) this is where the e-mail is broken down into packets of data and adds the source and destination ports to the packets as well as the sequence number; that way if a packet gets there quicker than another packet that was before it then the packets will be placed back in order. Then it goes to the network layer also known as the IP Layer, this is where the source and destination IP addresses are added, this is so the packet knows where to be delivered to. After the IP Layer the link layer adds the hardware addresses to each packet, the hardware addresses are also known as MAC Addresses, and puts them on the cable.
At home Bob uses his internet service provider to access the web. He uses this is access the Amazon webpage, using the HTTP protocol. The request is from the client to the host server. The client requests the web page from the application layer and the moves it way down the TCP Layer which adds a port number, which will be port 80 due to it being a web page, it will also add the sequence number to the packet of data that it has been split into. The packet is then passed to the IP layer where the source IP address is and where the destination IP address is. The packet is then once again passed down to the link layer where the MAC address is added so that the packet is added to the cable, for it to be sent. The packet is now sent across the local area network, which is a network that computers and other servers use to relay information. The local router for this local area network reads the address that the packet has and routes it to the right destination. When the packet arrives at the client server it goes up the layers back to the application layer. At the network layer the IP addresses are removed and then the transport layer reconstructs the data to be looked at on the application layer, which in this case is the web page Amazon. The web page is shown in the internet, which is a network of networks that allows a number of computers to gain information from other computers around the world. The packets have to make their way through the firewall to get to the web browser to show the page. The firewall stops untrusted data from gaining access to your computer and your files. You’re home network works like a sort of intranet as it only allows you to gain access to files and information on that network but also allows you to access information on the extranet. The packets that bob has sent will be processed to be directed to port 80 which is where all web pages are sent. Once the web page has been found the data is sent back along the path (through all the layers in the TCP/IP stack) back to the client (Bob). Bob buys a present for Alice on the Amazon website, he then uses the POP3 protocol to check for any new mail, POP3 is used for receiving e-mails and SMPT is used for sending e-mails, after checking for new e-mails he sends Alice an e-mail about the present, this will then go through all the layers and through port 25 which is the standard e-mailing port.
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