The External side of the Zeroshell has several external IP address that is assigned to us by the Internet Provider. I use to have these IPs on IPCOP as Alias.
the Router is not doing NAT.Router is not doing NAT.
Inside interface has VLAN 1, VLAN2, VLAN3, and the Mail server is on VLAN1 tagged on our switchs. the Mail server can access the internet fine while on VLAN1.
Hope that this helps.
BTW: VLAN1 is tagged on the ZeroShell ETH00.