You say the host IP addresses are like this:
- Host OS (Windows 7):
192.168.16.100
- Guest OS (Ubuntu):
192.168.56.101
- Mac OS:
192.168.16.102
If your guest OS is set to the 192.168.56.x
subnet that would be a host-only adapter in Virtual Box. Which is a great setup, but not so great for what you are trying to achieve. This simplest thing you could try to do is change that adapter from host-only mode to bridged mode.
When in bridged mode, the guest OS would be reaching out to your network’s router so it would be on the 192.168.16.x
subnet or whatever subnet your larger LAN is a part of. How to assign an address would be up to you, but if your router does DHCP you could assign a reserved IP address (static via DHCP based on MAC address) to this guest OS machine. Or—of possible—just see if you can safely assign a static IP address without too much hassle to the guest OS.
That said, if I were you I would prefer to do the following: leave the host-only adapter as-is but create a new, third network adapter in VirtualBox that would be the bridged adapter. That way your guest OS retains the host-only adapter connection on 192.168.56.101
, but you then gain an additional bridged adapter that is a part of the 192.168.56.x
subnet. That way to get the best of both worlds.