8.1 Protocols[an error occurred while processing this directive]
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.
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.
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.
8.2 Formats[an error occurred while processing this directive]
Formats are different from protocols, although they are often confused. Protocols are used for transferring information, but formats define the information format. For example, http is a protocol often used for transferring files. The most common file format transferred with http is html, although it can transfer other types of files.
Hyper Text Markup Language (HTML) is the defacto standard format for information on the World Wide Web (WWW). It basically allows text to be generally formatted with embedded images. When viewed, the display is adjusted to suit the browser. These documents also include hypertext links that allow a user to go to another HTML page, or download a file by ’clicking’ on linked text. Recently there have been many additions that allow more control over the ’look-and-feel’, but result in larger files and less portability. HTML files can be created with programs, or edited by hand.
A simple HTML file is shown in Figure X.X. It uses tags to define the beginning and end of definitions. The entire document begins and ends with the tags ’<HTML>’ and ’</HTML>’. The body of the document begins and ends with ’<BODY>’ and ’</BODY>’. Highlighted text is between ’<Hx>’ and ’</Hx>’, where ’x’ varies from ’1’ for the boldest to ’5’ to the lightest. Lists can be defined with the tags ’<UL>’ and ’</UL>’, and each point in the list begins with ’<LI>’.
The file in Figure X.X add a few features to the previous example. The first part is a header section that defines a title of the document. This title will appear on the top bar of the browser window. An image will appear after the heading and before the list, the image displayed is called ’test.gif’. The two items in the list now have hypertext links. The first is to a file called ’other.html’, the second is to another web site ’www.cnn.com’.
8.3 Clients and Servers[an error occurred while processing this directive]
• Some computers are set up to serve others as centers of activity, sort of like a campus library. Other computers are set up only as users, like bookshelves in a closed office. The server is open to all, while the private bookshelf has very limited access.
4. Call the Information technology people on campus, and request an IP address. Also ask for the gateway number, netmask, and nameserver numbers. They will add your machine to the campus DNS so that others may find it by name (the number will always work if chosen properly).
8.4 Java[an error occurred while processing this directive]
8.5 CGI[an error occurred while processing this directive]