DNS?

Hi.
I have redhat 7 with apache and DNS working on the same machine. Now what makes the www on the address work for the websever? Like the internet the address is www.xxx.xxx. The only way I can get the www to work is by inserting it in the apache ServerName directive that way, but I dont think thats the proper way.I tried putting it in the DNS zone files as CNAME but that didnt work. I am confused.
Can you enlighten me?

Thanks……Pat