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Steps

1. Edit /etc/network/interfaces file
    This file describes the network interfaces available on your system and how to activate them.
2. It will look like this
   # The loopback network interface
   auto lo
   iface lo inet loopback
   # The primary internal network interface
   auto eth0
   iface eth0 inet static
   address a.b.c.d                     (a,b,c,d is the IP address of internal network)
   netmask e.f.g.h                     (e,f,g,h is the netmask address)
   
   ADD FOLLOWING LINES IN THE FILE
   
   auto eth1
   iface eth1 inet static
   address a.b.c.d                     (a,b,c,d is the IP address of external network)
   netmask e.f.g.h                     (e,f,g,h is the netmask address)
   gateway v.x.y.z                     (v,x,y,z is the IP address of the gateway)
   
   
3. Edit /etc/dnsmasq.conf
   Find the line
   #resolv-file=
   and change it to
   resolv-file=/etc/dnsmasq-upstream-resolv.conf
   where /etc/dnsmasq-upstream-resolv.conf contains the upstream name server entry.
   
   Open the file /etc/dnsmasq-upstream-resolv.conf and put the following line in it:
   nameserver <upstream name server ip>
   where <upstream name server ip> it the IP of upstream name server
4. To access external network from virtual machine running in this private subnet, it requires nat rules in IP table.
   Those rules can be added using preinstaller script called "install_nat.sh".
   On running this script, it will list the available network interfaces and it will ask you to insert external network interface. Once the input is provided, it will add required rules in the IP table.