Sometimes you need to open a port on your server, you want it to be recheable only from specific IP address, you can use Iptables for this:
iptables -I INPUT -p tcp -s 10.1.1.2 --dport 22 -j ACCEPT
In that case, you are opening ssh port only to IP 10.1.1.2, if you need to open DNS for your internal network.
iptables -I INPUT -p udp -s 10.1.0.0/16 --dport 53 -j ACCEPT
Once you have them added and opened for those IPs, you need to close the door for the rest of IPs
iptables -I INPUT -p tcp -s 0.0.0.0/0 --dport 22 -j DROP iptables -I INPUT -p udp -s 0.0.0.0/0 --dport 53 -j DROP