I'm testing a web application using Apache AB; I love it but my web hosting service does not: if I increase the concurrency level over 30/40 I start getting 503 errors. The reason (according to the web hosting support) is that all the requests come from the same IP; do you know any other similar tool which can also simulate concurrent requests coming from different IPs?
Do you think the rule adopted by the hosting service is correct? For Optical Fiber internet connections, having many people who share the same IP is not so uncommon.