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.Show iptables rules.
iptables -L -v
Rules for open port 22 to one IP
iptables -I INPUT -p tcp -s 0.0.0.0/0 –dport 22 -j DROP
iptables -I INPUT -p tcp -s 45.56.77.20 –dport 22 -j ACCEPT
Save iptables rules
apt-get install iptables-persistent
iptables-save > /etc/iptables/rules.v4
systemctl start netfilter-persistent.service
Delete rules
iptables -D INPUT -p tcp -s 0.0.0.0/0 –dport 22 -j DROP
iptables -D INPUT -p tcp -s 45.56.77.20 –dport 22 -j ACCEPT