Application servers and network devices use IP addresses to identify clients. The application server often caches information about clients coming from the same machine. Network routers try to cache source and destination information to optimize throughput. If many users have the same IP address, both the server and the routers try to optimize. Since Vusers on the same load generator machine have the same IP address, server and router optimizations do not reflect real-life situations.
The multiple IP address feature enables Vusers running on a single machine to be identified by many IP addresses. The server and router recognize the Vusers as coming from different machines and as a result, the testing environment is more realistic.
Note: The maximum number of IP addresses that can be spoofed per network card for Windows NT SP3 is 35 IPs; Solaris (version 2.5.1) up to 255 IPs; Solaris (version 2.6 and higher) up to 8192 IPs.