TCP/IP Historical and Special Electronic Mail Addressing
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TCP/IP electronic mail has been so successful that it is arguably the most important worldwide standard for electronic messaging. The widespread acceptance of e-mail is tied inextricably to that of TCP/IP and the Internet as a whole. Since most organizations want to be part of the Internet, they connect to it and use its technologies, including the Domain Name System that is the basis for TCP/IP e-mail addresses. In turn, the use of simple DNS-style e-mail addresses (user@domain) encourages further use of e-mail because people find it conceptually easy to decide how to send messages.
TCP/IP is not, however, the only electronic mail system around. Over the years several other networks have developed e-mail systems. Due to the fact that the Internet is the largest internetwork in the world, TCP/IP e-mail has often been used as a clearing house of sorts to link together some of these different e-mail mechanism. This is called gatewaying, and allows someone using a non-SMTP e-mail system to interact with someone using TCP/IP, and vice-versa. Gatewaying is complex, and one of the reasons is that e-mail systems use different ways of addressing mail. Let's take a look at a couple of these systems and how they interact with TCP/IP.
One of the earliest independent e-mail systems was the FidoNet, which has been around for a long time and is still in use today. FidoNet is a worldwide network connected using modems and proprietary protocols; it is in essence, a competitor to the global TCP/IP Internet. I put competitor in quotes because the two are not really comparable in terms of number of users and the kinds of applications they support, but they are similar in overall objectives: worldwide electronic communication.
FidoNet users are identified using four numbers that specify the FidoNet zone, net, node and point (connection point). These addressing elements are used for sending mail on this system, which again is completely distinct from TCP/IP. However, to allow communication between TCP/IP and FidoNet, the FidoNet administrators have set up a gateway system that allows mail to be sent to FidoNet using TCP/IP style domain names. This style of mapping was also used by other systems with proprietary mail address formats to allow them to interface with the Internet.
For example, if a user was on machine 4, node 205, net 141, zone 1 (North America), the FidoNet address would be 1:141/205:4. The equivalent domain name would be p4.f205.n141.z1.fidonet.org, and could be used for TCP/IP-style user@domain addressing.
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