open port linux command

Red Hat Enterprise Linux 3 / 4 / 5 and above
=> Old Red hat Linux version
=> CentOS 4 and above
=> Fedora Linux
Open port 80

Open flle /etc/sysconfig/iptables:
# vi /etc/sysconfig/iptables
Append rule as follows:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
Save and close the file. Restart iptables:
# /etc/init.d/iptables restart
Open port 110

Append rule as follows:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 110 -j ACCEPT
Open port 143

Append rule as follows:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 143 -j ACCEPT
Restart iptables service

Type the following command:
# service iptables restart
Verify that port is open

Run following command:
netstat -tulpn | less
Make sure iptables is allowing port 80 / 110 / 143 connections:
iptables -L -n

Refer to iptables man page for more information about iptables usage and syntax:
man iptables