How to block a whole IP subnet with fail2ban and iptables

iptables-allports24.conf

# Fail2Ban configuration file
#
# Author: Cyril Jaquier
# Modified: Yaroslav O. Halchenko <[email protected]>
#                       made active on all ports from original iptables.conf
# Modified: Andreas Behringer
#                       made fail2ban block whole /24 subnets
#                       put this file in /etc/fail2ban/action.d/
#

[INCLUDES]

before = iptables-blocktype.conf


[Definition]

# Option:  actionstart
# Notes.:  command executed once at the start of Fail2Ban.
# Values:  CMD
#
actionstart = iptables -N fail2ban-<name>
              iptables -A fail2ban-<name> -j RETURN
              iptables -I <chain> -p <protocol> -j fail2ban-<name>

# Option:  actionstop
# Notes.:  command executed once at the end of Fail2Ban
# Values:  CMD
#
actionstop = iptables -D <chain> -p <protocol> -j fail2ban-<name>
             iptables -F fail2ban-<name>
             iptables -X fail2ban-<name>

# Option:  actioncheck
# Notes.:  command executed once before each actionban command
# Values:  CMD
#
actioncheck = iptables -n -L <chain> | grep -q 'fail2ban-<name>[ \t]'

# Option:  actionban
# Notes.:  command executed when banning an IP-range. Take care that the
#          command is executed with Fail2Ban user rights.
# Tags:    See jail.conf(5) man page
# Values:  CMD
#
actionban = iptables -I fail2ban-<name> 1 -s <ip>/24 -j <blocktype>

# Option:  actionunban
# Notes.:  command executed when unbanning an IP-range. Take care that the
#          command is executed with Fail2Ban user rights.
# Tags:    See jail.conf(5) man page
# Values:  CMD
#
actionunban = iptables -D fail2ban-<name> -s <ip>/24 -j <blocktype>

[Init]

# Default name of the chain
#
name = default

# Option:  protocol
# Notes.:  internally used by config reader for interpolations.
# Values:  [ tcp | udp | icmp | all ] Default: tcp
#
protocol = tcp

# Option:  chain
# Notes    specifies the iptables chain to which the fail2ban rules should be
#          added
# Values:  STRING  Default: INPUT
chain = INPUT

Just a small modification to etc/fail2ban/action.d/iptables-allports.conf