how to accept only specific “subnets” using iptables?

Solution 1:

You just use CIDR notation like in your question

sudo /sbin/iptables -A CHN_PNTS --src 182.24.137.0/24   -j ACCEPT
sudo /sbin/iptables -A CHN_PNTS --src 182.24.138.0/23   -j ACCEPT

Remember to place them before your

sudo /sbin/iptables -A CHN_PNTS -j DROP

so that they are actioned correctly.

Subnet specifications are acceptable in the source.

sudo /sbin/iptables -A CHN_PNTS --src 182.24.137.0/24 -j ACCEPT 
sudo /sbin/iptables -A CHN_PNTS --src 182.24.138.0/23 -j ACCEPT