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Nessus has an evaluation license with a restriction on the number of IP addresses that you can scan, and a full license, bought for a certain length of time and without any restrictions on the number of IP addresses one can scan. A fully licensed version of Nessus is available at approximately $2,500 per scanner on the Nessus website:

Select the Edit option next to the Activation Code.
In the box displayed, select the type of Nessus in use.
In the Activation Code box, type your new activation code.
Select Activate.

Once done, Nessus downloads the plugins required and installs them automatically.