Canonical Name (CNAME) records function as aliases. zippo.com could be aliased to www.zippo.com using a CNAME. CNAMEs allow you to alias one host name to another, however, the host that you are aliasing to MUST have an A record. You cannot define an alias using another alias that does not eventually point to an address. Let's take an example of setting up a web server:
zippo.net. IN CNAME www.zippo.net. www.zippo.net. IN A 198.204.18.190
I cannot stress this next point enough:
WARNING!! USING CNAMES WITH MX RECORDS IS HAZARDOUS TO YOUR DOMAIN'S HEALTH! IF YOU ARE READING THESE PAGES FOR REFERENCE YOU SHOULDN'T BE USING CNAMES!!
The fastest way to get yourself fired from your job if you're new to DNS is to use CNAMES with MX records to point multiple mail exchanger records to one host name using CNAMEs. You CANNOT CNAME a host named in an MX record to something else. This will BREAK the mail service, AND prevent the zone from loading causing ALL DNS TO FAIL FOR ALL HOSTS INCLUDING YOUR WEB SERVER!!! Your e-mail will break, your website will break, your FTP server will break and anything else that is in that zone file will stop working because noone will be able to find the IP addresses of those devices.
If you're running true Microsoft Active Directory (no WINS), users won't even be able to log in unless they are on Windows XP or 2000 clients and the default login policy hasn't been altered to prevent login when the domain controller can't be reached.
As we said before: CNAME records are hazardous for inexperienced DNS administrators.