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7.2 Protocols
7.2.1 Mail Transfer Protocols
Sending and receiving email involves different protocols. Mail is sent using a protocol called Simple Mail Transfer Protocol (SMTP). Mail is retrieved with Post Office Protocol (POP) or Internet Mail Access Protocol (IMAP). All of these protocols are handled with programs listening on different ports on the server.
7.2.2 FTP - File Transfer Protocol
File Transfer Protocol (FTP) is a very old and well supported method for transferring files between computers. Advanced users will often use it with typed commands, but there are also hundreds of graphical clients that hide the typed commands.
7.2.3 HTTP - Hypertext Transfer Protocol
Hypertext Transfer Protocol (HTTP) is used for retrieving web pages from remote sites. It uses simple commands to get text files from the remote computer.
7.2.4 Firewalls and IP Masquerading
A firewall is a single computer that acts as a point of contact between an untrusted network (the Internet) and a secure network. The firewall computer will have two or more network cards that it will monitor differently. Generally it is set up to allow traffic to pass from the internal network to the outside world with greater freedom than it allows network traffic to enter the secure network. This allows computers behind a firewall to get access to outside computers. Any requests they make are mirrored to outside computers, who may then respond to the requests. Requests from computers behind the firewall all seem to come from the firewall itself. There is no way for an outside computer to access a computer behind the firewall.
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