Domain Names Part 2: How DNS Works Continued

A vs. MX Records

Typically, hosts have both "A" records and "MX" records. The "A" records work as described above, and are used in determining targets for web connections, FTP, telnet, and other such services. "MX" records, though, deal with with mail delivery.

MX records serve two purposes. First, they allow you to specify a different target machine for electronic mail than for other Internet services. So, you can send all FTP, web, and telnet requests for ahref.com to one machine, while sending email to another machine elsewhere. This can be helpful in splitting load among your Internet servers, and splitting administration requirements.

Second, MX records let you specify several hosts to accept email for your domain. Because email is asynchronous, you don't want it to bounce if the main target machine is temporarily unavailable; the target machine might become available in a few hours, at which time the mail should come through. By specifying multiple MX records, you can have email spooled on a backup machine somewhere while waiting for your main mail machine to come back up. You do this by specifying an MX number for each mail host; mail will be delivered to the lowest-numbered currently available machine. If the lowest-numbered machine is not currently available, it will be delivered to a machine with a higher number, which will "spool" the mail, and deliver it to the appropriate, main, mail machine when that machine becomes available.

Next Time

So far, we've covered how administration of the Domain Name System is changing hands, and how DNS works from a technical standpoint. In our next article, we'll cover how several companies are diminishing the importance of the Domain Name System with proprietary solutions to the problems of Internet addressing.

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Edward Piou is an ahref.com producer and runs ep Productions, Inc., a development company based in the Washington, D.C. area.

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