Adding static addresses on single network adapter

[CA1264996]

I have acquired and installed a Proliant ML350T server in a business preexisting LAN.
The operative system OEM is Windows Server 2003 SBS Premium.
The server has an only card adapter with one static private address.
It is connected to internet through a Cisco 2600 router. I need add a further ( or more ...) static private address ( same LAN / in DHCP exclusion list ) on that build a virtual web server. In a following phase, through NAT (router) , the new private address comes combined to a static internet address. When I try to add a new address ( TCP/IP-Properties / Advanced...) SBS automatically sets this new one as primary address (!?) and if, through wizard or not, I try to re-change as primary the old address It deletes the new additional address (???).
I have followed the information contained in a Microsoft doc " Publishing a SharePoint Web Site Accessible to External Users ".
In practice I can’t get a "multi-homed" configuration of the only adapter of the server.
Can You help me? Thanks