Win IP Config solved this hairy problem for me in Windows 7!
It seems like there are a lot of people who don't understand why having a simultaneous DHCP and static IP would be necessary or convenient.
In my case, I have to configure machine control systems that communicate over multiple subnets, between engineering computers that must do the same, while maintaining the ability to access files on the corporate servers, and access the internet for drivers, software updates, etc.
This particular project has a Siemens PLC communicating with 6 Caterpillar generators using MODBUS TCP on xxx.xxx.1.1 thru xxx.xxx.1.6, with the PLC assigned xxx.xxx.1.7, and the HMI on xxx.xxx.1.8.
My router is set to assign DHCP addresses on xxx.xxx.47.xxx, connecting me to the corporate network via NAT.
The MODBUS simulator is running on an XP machine that is assigned an address via DHCP from my router xxx.xxx.47.xxx, plus the six generator IP addresses (xxx.xxx.1.xxx), plus an additional IP address to make it accessible from the xxx.xxx.1.xxx subnet.
The HMI simulator is running on a Win 7 machine that also receives a DHCP address, but required the additional xxx.xxx.1.xxx address to make it visible to the Siemens PLC and the Win XP machine while preventing the TCP/IP stack from binding to any of the addresses reserved for the generators.
Additionally, I need DHCP addresses to be available for ease of administrating a program called "Input Director" which allows me to share clipboards and control any of eight screens, from any mouse attached to any of my 6 engineering PC's.
The registry hack made setting up dual DHCP/Static IP's easy on the XP machine, but Win 7 was a different story.
While all of the above was certainly possible by assigning static IP's across the board, the combination of applications I use requiring network access makes it impractical to manage quickly.
Win IP Config saved the day, thanks for the suggestion!!!