Default website of a web server by viewing by ip address

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http://serverfault.com – i have a centos + webmin + virtualmin installed. The web server is hosting multiple website. For example: domainA.myftp.org domainB.myftp.org domainC.myftp.org all domain sharing same ip address but when i directly type the public ip address, the browser will display domainA.myftp.org where can i set the website directory of an ip address? It this a dns or Apache virtual host or other issue? anyone knows? (HowTos)