By default iptables is the firewall that is enabled on linux systems. It is very simple to open ports without totally disabling the iptables service. Here are simple steps:
iptables -I INPUT -p tcp --dport <port> -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -I OUTPUT -p tcp --sport <port> -m state --state ESTABLISHED -j ACCEPT
Example to open http port – 80:
iptables -I INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -I OUTPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT