You have most likely configured your guest's network adapter in NAT mode.
Shut your guest down.
Go to the Guest's network settings and in the Adapter settings change the
Attached to: NAT to
Attached to: Bridged Adaptor.
Start your guest
Your guest should now pick up an address from your main network.
To find the adaptor settings, in the Oracle VM VirtualBox Manager click on your guest. On the right you will see a series of panels. Click on the Network Panel. A pop-up will apear with the settings in.
Here is a blog post with some images attached: http://house9.blogspot.co.uk/2011/04/ping-guest-os-from-host-os-with.html