Setting up 2 VM subnets each with 2 virtual machines and add routing between them in VirtualBox

I'm going to create 4 virtual machines running on VirtualBox and 2 Subnets with 2 VMs per each subnet and add routing between those 2 subnets.

If you are new to VirtualBox it's better to go thru the network types available in VirtualBox [1]

First you need to enable a new network adapter and attach the network to "Internal network". use the same internal network name for each VM.
After enabling start the VMs and configure each VM as follows in the interface configuration
/etc/network/interfaces

Subnet 01 contains VM01 and VM02
10.1.0.0/24

Subnet 02 contains VM03 and VM04
10.1.1.0/24

Following shows the configuration which should be added to Subnet 01's one VM.

iface eth1 inet static
         address 10.1.0.1
         netmask 255.255.255.0
         network 10.1.0.0
         broadcast 10.1.0.255

likewise configure each VM with the relevant configuration

after that stop the interface and start the interface using following commands
sudo ifdown eth1
sudo ifup eth1

you might sometimes need to restart the network-manager service too. in that case use command
sudo service network-manager restart

After configuring subnets you will be able to ping from/to VM01 from/to VM02 and  from/to VM03 from/to VM04

By executing following command you will be able to add routing between the subnets

route add -net 10.0.1.0/24 dev eth1 (this is for subnet 02 machines)

you can view the routing table by executing "route -n"

Now you will be able to communicate with each and every node in the two subnets


[1] https://www.virtualbox.org/manual/ch06.html