Although there are many different types of DNS records, most of these record types aren't commonly used.
You create a new host entry with A and PTR records by doing the following: You specify host aliases using CNAME records.
The mail server with the lowest preference number has the highest priority and is the first to receive mail.
If mail delivery fails, the mail server with the next lowest preference number is tried.
Aliases allow a single host computer to appear to be multiple host computers.
For example, the host gamma.can be made to appear as ftp.