Accessing your Virtualbox Guest from your Host OS

1. If your guest machine is running, shut it down first.
2. Click on File->Preferences in the VirtualBox menu-bar.
   
3. Select the Network option from the side menu and click on the Host-only networks tab.
   
4. The default options for the newly-created Host-only network should be fine. If not, you can add the following data manually, by clicking on the Edit button in the DHCP Server tab.
   • Server Address: 192.168.56.100
   • Server Mask: 255.255.255.0
   • Lower Address Bound: 192.168.56.101
   • Upper Address Bound: 192.168.56.254

   Don't forget to check the Enable Server option.

   
   
5. Now save all the settings in Preferences.
6. Now open up the settings of your Guest machine and navigate to the Network option from the side menu and click on the Adapter 2 tab.
   
   
7. In the Attached to section, click on the dropdown and choose Host-only Adapter and in the Name section nested in it, choose the newly-created Host-only network.

   Don't forget to check the Enable Network Adapter option.

   
8. Save these settings and boot into your Guest machine.
9. After logging in, type ifconfig. Note the new IP, it should be under a new interface like eth0 or vboxnet0. Now you can use this IP to SSH, view the webpages on your machine's Apache Server, etc..

Thanks to  2Ubuntu