What open server ports can I close? Which ones are necessary?

view story

http://serverfault.com – I'm trying to strengthen my server's security. I have a pretty standard CentOS VPS setup. Below is a list of both TCP and UDP ports configured in my CSF firewall. These ports were open by default since I got my server. I have compiled as much information about these as I can find. Some ports are obvious to me and I should keep them open (HTTP, POP, cPanel, etc). However there are a lot of ports and (and port ranges!) that I have no idea about and if they need to remain open. Which of the following ports can I close? Are there any risks in closing them? 20 FTP - Planning to disable FTP any (HowTos)