Corporate.Net internetRx By Chris Lewis Q: Our Internet service provider (ISP) has set up all of our network devices for Internet access. I know that packets are delivered on the Internet according to the destination IP address number. How is it that my users can send e-mail to someone simply by specifying a character-based e-mail address? A: To deliver e-mail across the Internet, you must use Simple Mail Transport Protocol (SMTP) and Domain Name Service (DNS). Generally, there is one machine (an SMTP hub) on an intranet that is respon sible for passing mail between the internal network and the Internet. This hub will use SMTP processes to exchange mail with Internet destinations. A DNS server has two purposes in this process. First, it holds information usable by other DNS servers on the Internet regarding the name of the local mail server. Secon d, the server resolves names to IP addresses so that packets can be addressed properly. SMTP calls for a Mail Transport Agent (MTA) and a Mail User Agent (MUA). In the world of Unix hosts, the most common MTA is sendmail. A popular MUA is QUALCOMM's Eudora Pro, which acts as the user interface on the client computer. In the world of PCs, a Post Office Protocol version 3 (POP3) client program runs natively on desktop computers to provide a PC user interface. So, the real question is for outbound mail messages: How does an MTA decide which IP address e-mail is destined for, given only a string of text (user and domain name) to define the site? Let's look at a fictitious organization tha t uses the domain destination.com and someone on the Internet who wants to send an e-mail to a user within this domain. Two processes occur. First, the DNS server for the destination.com domain will have one or more mail exchange (MX) entries that specify the host name and IP address of the e-mail machine for this domain. These MX entries then can be queried by other DNS servers on the Internet to determine the host name for the destination.com domain's e-mail server. Second, DNS services at the location originating the e-mail resolve the host name to an IP address. In the configuration of the destination.com domain's DNS server, there will be MX entries that look something like this: destination.com. IN MX 10 mailserver.destination.com. This entry identifies mailserver.destination.com as the machine to deliver mail to for the destination.com domain. The number 10 indicates a preference for this entry over other possible entries for the domain (the lower the number, the greater the preference). Once your mail exchanger's host name is known, the MTA then must translate that into an IP address. On your DNS server, that entry will look like this:mailserver.destination.com. IN A 192.155.15.10 Therefore, any user who sends a message addressed to user1@destination.com will have the message forwarded to mailserver.destin ation.com. The system originating this e-mail will then use DNS to resolve mailserver.destination.com to an IP address, and it will send the packet out on the Internet. Once the message has arrived at mailserver.destination.com, the specific e-mail package at the other end will deliver the mail to the desktop, using its internal processes.

Private Dial-Up Networking Gets a Virtual Workout by Mike Fratto Updated July 8, 1997