DNS Introductions:

Table of Contents

Understanding DNS Records - An Example
A Records
CNAME Records
MX Records
Conclusion

DNS Primer

http://www.clarkconnect.org/help/dns/dnsprimer.html

The Domain Name System (DNS) is not difficult to understand, but you do have to take a little time to learn some of the rules.

Basically, DNS is used on the Internet to map names to IP addresses. A mapping is simply an association between a system name (e.g. www.clarkconnect.com) and a system's IP address (e.g. 216.40.215.42). However, different services use different types of mappings or records. The example below will walk you through a typical setup for a typical server.

Understanding DNS Records - An Example

Let's go through a typical example. You have just registered sample.com as your domain.

You use your primary machine as a web server and a mail server.
The web server is to be accessible at http://sample.com and http://www.sample.com.
You have a password protected MP3 web site at mp3.sample.com.
Just in case, you also want to specify a backup mail server.
You have a remote office with a cable modem connection, and you want to use remote1.sample.com as the domain name.
You have a hot tub connected to the Internet with a static IP - 128.2.11.43. You can pre-warm the hot tub over the Internet by sending an encrypted signal to this IP address.

The DNS configuration for this setup is shown in the table below. Don't worry about the details yet… keep reading!

A Records
sample.com 216.40.215.42 (your primary server)
hottub.sample.com 128.2.11.43
CNAME Records
remote1.sample.com cr624222-a.someisp.com
mp3.sample.com sample.com
www.sample.com sample.com
mail.sample.com sample.com Bad Don't do this... see below
MX Records
sample.com sample.com
sample.com mxbackup.pointclark.net

A Records

The bread and butter behind the DNS system is the A record. The A record (address record, or host record) maps a domain name to an IP address on the Internet. All systems configured in your account are - by default - A records.

In our example, you would set your primary system for your domain: sample.com. The IP address (216.40.215.42) for sample.com will be automatically updated if you have a dynamic IP! For your hot tub, you type in the static IP address (128.2.11.43) to be associated with hottub.sample.com.

So, you have two names mapped to IP addresses (A Records):

sample.com to 216.40.215.42
hottub.sample.com to 128.2.11.43

CNAME Records

CNAME records (Canonical Name records) act as aliases for hostnames. Instead of mapping a domain name to an IP address (an A record) you map a domain name to another domain name! In the example, you have:

mp3.sample.com to sample.com
www.sample.com to sample.com
remote1.sample.com to cr624222-a.someisp.com

What are the advantages of CNAMEs? You can map multiple domain names to one - sometimes dynamic - IP address. mp3.sample.com and www.sample.com will now be associated with sample.com's IP address (our first A record). In the case of the remote office, you change a not-so-easy-to-remember-super-long domain name into something better.

MX Records

The Mail eXchanger record (MX record) tells mail systems how to handle mail that is addressed to a particular domain. Like CNAME records, the MX record maps a domain name to another domain name.

In the example, we use our primary machine as a server for mail to xyx@sample.com. Every MX record is tagged with a priority number. The MX record with the lowest number is the primary mail server. If the primary server is unavailable, the backup mail server (also called a "secondary mail server") will queue the mail. You can use the MX Backup Service to use Point Clark Networks' mail servers as a backup.

Domain Priority Mail Server
sample.com 10 sample.com
sample.com 100 mxbackup.pointclark.net

In our example, sample.com is your main mail server. If someone spilled coffee on this server and knocked out the hard drive, then mxbackup.pointclark.net would automatically queue xyz@sample.com mail until sample.com comes back online.