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Originally Posted by License Agreement

# vBulletin license grants you the right to run one instance (a single installation) of the Software on one web server and one web site for each license purchased. Each license may power one instance of the Software on one domain. For each installed instance of the Software, a separate license is required. Modifications to the Software or database to circumvent the one-license-one-board rule are prohibited.
# The Software is licensed only to you. You may not rent, lease, sublicense, sell, assign, pledge, transfer or otherwise dispose of the Software in any form, on a temporary or permanent basis, without the prior written consent of Jelsoft.

http://www.vbulletin.com/order/license_agreement.php