This is possible in Windows Vista, but you might have to use all-static IPs (no DHCP) if I'm remembering correctly. Follow these steps:
- Open the Control Panel
- Open the "Network and Internet" applet
- Click on "Network and Sharing Centre"
- Click on "Manage network connections" (below the "List of tasks" heading)
- Right-click on the appropriate NIC
- Click on "Properties" (usually at the bottom of the pop-up menu that will appear)
- Double-click on "Internet Protocol Version 4 (TCP/IPv4)" protocol
- Click on the "Advanced" button
In the Advanced settings, you will want to find the screen that looks like this one from XP (the one in Windows Vista looks almost exactly like this but with the current Windows Vista GUI theme), which will let you specify multiple IP addresses by clicking on the "Add" button:
As you can probably see, you can specify a subnet mask. This is a useful feature because different subnets could have different masks (for the most-part, the mask reveals the size of the subnet that's directly accessible from your computer).