Your System is going use as a router for 172.

Do the following steps as:

echo “1” >/proc/sys/net/ipv4/ip_forward

vi /etc/sysctl.conf

net.ipv4.ip_forward=1

/proc is the virtual filesystem, containing the information about the running kernel. To change the parameter of running kernel you should modify on /proc. From Next reboot the system, kernel will take the value from /etc/sysctl.conf.