First off, CentOS uses the firewall IPTables by default. To go in and open up ports you will need to start off as root :

sudo su –

You will be prompted for your root password. Go ahead and enter it and hit enter.

It’s always a good idea to turn IPTables off before you start :

service iptables stop

Now you will need to run the following command based on if you want to open a TCP port or a UDP port :

iptables -I INPUT -p tcp –dport 12345 –syn -j ACCEPT
Make sure to change 12345 with the port you want to open

iptables -I INPUT -p udp –dport 12345 -j ACCEPT

Now you need to save the changes you’ve made to IPTables :

service iptables save

And finally, start the service back up :

service iptables start