You'll have to establish a tcp connection with the server before you'll be able to send an http packet that will be received. Its a bit of work to build up from the ip level what you might need to do this kind of testing.
There are tools that will generate http sessions for you though. They can usually be configured to open a particular number of simultaneous http sessions as well, if this is what you are trying to do when you mention changing the source ip. Something like http://code.google.com/p/httperf/ or http://httpd.apache.org/docs/2.2/programs/ab.html might do it.