When a test is carried out each IP address in a subnet is used sequentially. For example IP 126.96.36.199 is used as a source. The next simuser will use 188.8.131.52 then the next simuser will use 184.108.40.206 and so on.
In the case if a simuser is reusing an IP every 3 seconds a client source IP address, check the DNS timeout. The DNS "First Retry interval" in Client Profiles | DNS is set to 3000ms (default). Adjust the timeout to compensate for any delayed reply from the DNS.