Post Office Protocol version 3 (POP3) and Internet Message Access Protocol version 4 (IMAP4)

Both POP3 and IMAP4 are mechanisms for downloading, or pulling, email from a mail server. They are necessary because, although the mail is transported around the network via SMTP, users cannot always read it immediately so it must be stored in a central location. From this location, it must then be downloaded, which is what POP3 and IMAP4 allow you to do.

One of the problems with POP3 is that the password used to access a mailbox is transmitted across the network in clear text. That means if someone wanted to, he could determine your POP3 password with relative ease. This is an area in which IMAP4 offers an advantage over POP3. It uses a more sophisticated authentication system, which makes it harder for someone to determine a password.


The function of Telnet is to allow the establishment of sessions on a remote host. A user can then execute commands on that remote host as if he were physically sitting at the system. Telnet is widely used to access UNIX and Linux systems, as well as to administer some managed networking equipment such as switches or routers. Telnet uses TCP as a transport layer protocol and functions at the application layer of the OSI model.

Secure Shell (SSH)

Secure Shell (SSH) is a secure alternative to Telnet. SSH provides security by encrypting data as it travels between systems. It also provides more robust authentication systems than Telnet.

Although SSH, like Telnet, is primarily associated with UNIX and Linux systems, implementations of SSH are available for all commonly used computing platforms including Windows and Macintosh. As discussed earlier, SSH is the foundational technology for the Secure File Transfer Protocol (SFTP).