Centos firewall blocking incoming connections

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http://serverfault.com – After installing Centos 5.8 I could not pick up any open ports using nmap. So I ran the command #iptables --flush and all started working. However, after about 2 days I could not connect again and had to run the command. 1)How can I permanently ensure that my ports are open 2)What could be causing them to be shut in the first place since I did not touch the iptables? (HowTos)