Block ports on Linux

To block port 80 (HTTP server), enter (or add to your iptables shell script): 

# /sbin/iptables -A INPUT -p tcp --destination-port 80 -j DROP
# /sbin/service iptables save

 Block Incomming Port 80 except for IP Address 1.2.3.4 

 # /sbin/iptables -A INPUT -p tcp -i eth1 -s ! 1.2.3.4 --dport 80 -j DROP

To block outgoing port # 25, enter:

# /sbin/iptables -A OUTPUT -p tcp --dport 25 -j DROP
# /sbin/service iptables save

You can block port # 1234 for IP address 192.168.1.2 only:

# /sbin/iptables -A OUTPUT -p tcp -d 192.168.1.2 --dport 1234 -j DROP
# /sbin/service iptables save

To unblock ports. 

 /sbin/iptables -A INPUT -p tcp --destination 12375 -j ACCEPT
 /sbin/iptables -A OUTPUT -p tcp --destination 12375 -j ACCEPT

/sbin/service iptables save