This is a new Mercury KB article by Srinivas Saripalli...
When testing using Web and WinSocket protocols, by default Vusers on each host machine use the host's (agent's) IP address. Since a particular server keeps serving a specific IP address, load tests would reflect network router caching and other optimizations rather than the real-life situation where different users arrive to the server with different IP addresses. This is especially needed when the hardware configuration under test balances load across a "farm" of several (web or database) servers. "IP spoofing" enables each host machine to "spoof" the server into thinking it is using many different IP addresses.
First, internal IP addresses (such as 192.168.1.1) may need to be allocated by the organization's Network Administrator unless you're working in a closed network. The IP Wizard program (ipwizard.exe in LR bin) is used to define multiple IP addresses on each load generator host. However, the hosts must use static IP addresses rather than DHCP to dynamically assign addresses. Note: IP addresses include two components, a netid and hostid. The submask determines where the netid portion of the address stops and where the hostid begins.
When a host machine is re-started, routing tables are updated with the new addresses.The IP Wizard program creates a batch file (nt_routing.bat for Windows and unix_routing.bat for UNIX machines) and .ips file which updates IP tables. To enable multiple IP Addressing At the LoadRunner Controller, pull down menu item Scenario, check "Enable IP Spoofer" (In earlier versions, "Enable Multiple IP Addresses"). This enables "Multiple IP address mode" in the Options dialog General tab, which appears after "Expert mode" is checked on the Tools drop-down menu. IP allocation should be consistent with the way users are running.
After turning IP spoofing on or off, for the setting to take effect, the Load Generator needs to be disconnected and reconnected again.