Thanks for the help from you all , now am back to the same issue internet connection sharing using NAT .I use the following command it work for me for an hours ..
sudo iptables -A FORWARD -o eth0 -i eth1 -s 192.168.0.0/24 -m conntrack --ctstate NEW -j ACCEPT sudo iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT sudo iptables -A POSTROUTING -t nat -j MASQUERADE
Edit /etc/rc.local and add the following lines before the "exit 0" line:
sudo iptables-save | sudo tee /etc/iptables.sav
iptables-restore < /etc/iptables.sav
After restarting my server NAT is not working .client system can ping to my server through the server's eth1 ip but Internet is not accessible to the client pcs .I think its my fault but I cant rectify what is the issue ..please any give me your configuration ....and also I wish to implement the attached structure in my institution if it any wrong on there please suggest me to change .......
sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"