echo "1" >/proc/sys/net/ipv4/ip_forward
vi /etc/sysctl.conf
net.ipv4.ip_forward = 1
If you want to use the Linux System as a Router to make communication between different networks, you need enable the IP forwarding. To enable on running session just set value 1 to
/proc/sys/net/ipv4/ip_forward. As well as automatically turn on the IP forwarding features on next boot set on /etc/sysctl.conf file.