The Alexa protocol is far more detailed to make this really work for any site worth more than a few dollar.
Alexa checks useragent (browser, os etc), alexa toolbar version, number of sites you visited, delay between visits, a generated session id and more.
There would be no reason to use such a detailed protocol if they'd not regulary test their ranks for fakers.
But it is true, the MAIN factor of alexa is the unique IP.
A secret for free: you need about 800 unique IPs to be able to gain a rank of 999,999.
I do not know how alexa handles obviously faked rankings, but I'd not risk a ban faking a ranking that way if the site you want to be boosted is a long-term investment.
If you just need a high boost for a short time, why not.