Will this script work ?
#!/bin/bash
IP=$(/sbin/ifconfig eth1| grep 'inet addr:'| cut -d: -f2|awk '{print $1}')
MASK=${IP%.*}.0
echo '1' > /proc/sys/net/ipv4/ip_forward
iptables -I FORWARD -s $MASK/255.255.255.0 -j DROP
iptables -A FORWARD -i eth1 -o wlan0 -s $MASK/24 -m state --state NEW -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A POSTROUTING -t nat -j MASQUERADE