intermediate nodes, called message transfer agents (MTAs). MTAs are used in large companies to serve multiple local servers for external e-mail exchange. When mail enters the mailbox identified by the destination address, the destination user may access and extract it from the mailbox. For that she may use Post Office Protocol (POP) or Internet Message Access Protocol (IMAP) to get e-mail to her terminal for local processing. 6.6.10.2 FTP FTP is an application protocol for the transfer of files among different com- puters. The objective is to provide users access to files in other computers. The user is able to view a remote computer's file catalog and request the transfer of any file of interest [5]. Introduction to Telecommunications Network Engineering To copy a file, an FTP user establishes a TCP connection to the FTP server (port 21) for file transfer control. This is actually a Telnet connection via which commands for establishment of another TCP connection, used for actual data transfer, are given. Now a command, such as get <file>, may be given to copy a file from the FTP server to the user's computer. When using a browser's graphical user interface, we just click a button and Telnet and FTP do the actual work. 6.6.10.3 HTTP HTTP is a standard protocol used on the Web. It is based on client-server communications between a Web browser (client) and a Web server. Its opera- tion and how it is used in Web surfing are introduced in Section 6.6.11. 6.6.10.4 Telnet The Telnet protocol provides a standard method for communications between terminal and terminal-oriented processes on a host computer. With the help of Telnet a user can log on to a remote host computer instead of his desktop PC. Telnet is based on the network virtual terminal (NVT) concept, which represents a standard terminal and a set of services needed in the terminal ses- sion. Several application protocols, such as SMTP, are based on Telnet. 6.6.10.5 DNS The Internet's IP addresses in binary, dotted decimal, or hexadecimal format are not especia