How to block/unblock an IP from Linux?

In order to block an IP on your Linux server you need to use iptables tools (administration tool for IPv4 packet filtering and NAT) and netfilter firewall. First you need to log into shell as root user. To block an IP address you need to type the iptables command as follows:

Syntax to block an IP address under Linux

iptables -A INPUT -s IP-ADDRESS -j DROP

Replace IP-ADDRESS with your actual IP address. For example, if you wish to block an ip address 65.55.44.100 for whatever reason then type the command as follows:

# iptables -A INPUT -s 65.55.44.100 -j DROP

If you have IP tables firewall script, add the above rule to your script.
If you just want to block access to one port from an ip 65.55.44.100 to port 25 then type command:

# iptables -A INPUT -s 65.55.44.100 -p tcp --destination-port 25 -j DROP

The above rule will drop all packets coming from IP 65.55.44.100 to port mail server port 25.

CentOS / RHEL / Fedora Block An IP And Save It To Config File

Type the following two command:
# iptables -A INPUT -s 65.55.44.100 -j DROP
# service iptables save

How Do I Unblock An IP Address?

Use the following syntax (the -d options deletes the rule from table):
# iptables -D INPUT -s xx.xxx.xx.xx -j DROP
# iptables -D INPUT -s 65.55.44.100 -j DROP
# service iptables save