Internet Applications

FTP

You must be familiar with the File Transfer Protocol (FTP). This protocol is used as a simple method of transferring files between computer systems. FTP actually requires two TCP ports for communication a data port and a command port. The command port communicates over TCP port 21 and the data port normally uses TCP port 20.

E‐Mail

Email is a shortened version of the two words 'electronic' and 'mail' and can be considered the electronic version of the letter. It is one of the most used and popular services on the Internet. Email enables messages to be transferred from an individual to another individual or from an individual to a group of people. Documents (audio, video, pictures etc.) can be attached to email messages and sent with the message. Email can be sent and viewed various ways ‐ the most common through a computer program, such as 'Outlook'. Email can be sent to anywhere in the world and viewed whenever the recipient logs onto the Internet and checks their ‘mailbox’ where emails are stored.

HTTP

The Hypertext Transfer Protocol (HTTP) is one of the most widely used protocols on the World Wide Web. Surfers use a Web browser to request content from the web site. The browser, in turn, communicates to the Web server using HTTP commands. Web content is then returned from the Web server to the Web client using the HTTP protocol. HTTP also operates at the application layer protocol and used TCP and UDP ports 80 for normal communication and uses TCP and UDP ports 443 when communication over SSL.

DNS

To translate a typical URL address typed into a Web browser, there must be another layer to convert it into the IP address. This translation is usually accomplished by Domain Name Server (DNS) servers that are installed within organizations, at ISP’s and at key points within the internet structure. DNS operates within the application layer and allows clients to access these various DNS servers to perform the translation. The distributed nature of this information store allows users to type a domain name (eg. Microsoft.com) into a Web browser and get the same response regardless of where the user is located.

Typically when a user is trying to translate a domain name into an actual IP the DNS server performs a query over UDP port 53 to the DNS server. UDP allows the DNS query to be quick and efficient. To allow the reliable exchange of this information, DNS switches to TCP for communication and uses port 53.

P2P File sharing

In peer to peer network there are no dedicated servers, and there is no hierarchy among the computers. All the computers are equal therefore knows as peers. Each computer functions as both a client and a server, and there is no administrator responsible. The user at each computer determines what data on that computer is shared on the network.

Some common P2P file sharing applications are,
• Ares
• Bit Torrent
• Fast Track