To block, unblock or check IP addresses with iptables in Linux

To block or check IP addresses with iptables

The format to block and IP address is as follows:
iptables -A INPUT -s BAN-IP-ADDRESS -j DROP
iptables -A INPUT -s BAN-IP-ADDRESS/MASK -j DROP

To block an IP address, use the following;
iptables -A INPUT -s 192.168.254.100 -j DROP

To view all blocked IP addresses, do;
iptables -L INPUT -v -n
OR
iptables -L INPUT -v -n | less
OR (especially if blocked with fail2ban) **
iptables -L -n --line

To check for a specific IP address, do;
iptables -L INPUT -v -n | grep "192.168.254.100"

To unblock an IP address, do;
iptables -D INPUT -s 192.168.254.100 -j DROP

Alternatively to unblock an entry blocked with fail2ban (see ** above) do;
iptables -L -n --line
then find the IP address you wish to unban.
then do
iptables -D ruleName n
where ruleName is the rule name  associated with the banned IP
and n is the number associated with the banned IP

ref: https://www.cyberciti.biz/faq/linux-howto-check-ip-blocked-against-iptables/