Alpine linux virtualbox networking

Using virtualbox, I am trying to use Alpine linux (guest) as a firewall for my lubuntu (host). My physical network card (NIC) is eth0.

ISP WAN -> Alpine linux (guest) -> lubuntu (host) LAN

I am trying to get the ip from my ISP DHCP server but I had no success. I know that in virtualbox I have to use bridged adapter in eth0.
But in AlpineLinux (guest) do I have to create a vlan? Thats where I get confused.

ISP WAN -> DHCP -> Alpine linux (guest)

Links:

http://timita.org/wordpress/2011/07/...rt-1-preamble/
http://www.instructables.com/id/How-...-to-Protect-y/
http://brezular.com/2015/01/18/pfsen...wall-on-linux/

I was able to do this with two PCs like this:

ISP WAN --> Alpine linux firewall (PC1) --> Lubuntu LAN (PC2)

But now I am trying to do it with just one PC using virtualbox like this:

ISP WAN --> Alpine linux firewall (PC1) (Guest) --> Lubuntu LAN (PC1) (Host)