Port Forward from Windows 2008 R2 Host, to Ubuntu guest

[Rockym]

Hello,
I am running VirtualBox 4.3 on a Windows 2008 R2 server. The server is configured with multiple public IP addresses and I would like to use one of the addresses for the guest Ubuntu server I have running. I configured port forwarding from the IP address that I want to use but it would not work with port 80. It only worked if I set the host port to 8888 and forwarded it to port 80 of the Ubuntu server.
The Windows host I am using is a hosted server, so I only have remote access to it and no physical access. It is also hosting websites in IIS7 but not on the IP that I want to use.

Is there anyway possible for the VirtualBox NAT to be configured to use one of the public IPs bound to the server and forward its relevent ports (80,25,etc) to the guest server or is that not possible in Windows? I know it could probably be done by adding a second physical NIC but that isn't possible for me to do. Thanks.

[noteirak]

When you configure the port forwarding, you can specify which IP should be used to listen on.
But using NAT for this kind of setup is NOT the good way, as NAT is a very simplistic network mode which is designed for going out, not coming in.

If you need to do a server-based configuration, I would recommend switching to host-only or bridged, or install a proper OS that let you manage network easily and is a great host for VMs (Linux, BSD, etc).