Effective IP blocking with Iptables: A guide to secure Linux systems

iptables are recognizable to most system administrators. It has been around for a long and is enabled by default in the Linux kernel. We may use iptables to ban a single IP address, several IP addresses, or whole networks. This may be useful if you receive repeated port scans or witness failed unauthorized access in your log files. IP blocking is a more effective security measure.

Source: FOSS Linux