As I understand it the maximum allowed output power of a 2.4Ghz wireless device is 4 watts erip (36 dbm) so taking your typical wireless device to have an output of 100mw (20 dbm) that would mean that attaching an antenna that has a gain of more than 16 dB would exceed this legal maximum.

Why then is it possible to purchase parabolic aerials such as the one here
http://www.tp-link.co.nz/products/productDetails.asp?class=wlan&pmodel=TL-ANT2424B
that have a gain of 24 db. Unless there's something I'm not quite understanding here that would mean that an assumed input of 100mw (20 dbm) would equate to a focused output of 25.1 watts or 44 dbm.

If someone can enlighten me as to what's going on here that would be great as I am exploring options for a friend who is looking to setup a wireless connection to bridge two networks over long distance. Is this a bit of a grey area in regards to 2.4Ghz transmissions or are such 24db antennas sold on the assumption that their input power won't exceed 15.85mw (12 dbm) even though the parabolic antenna in question is speced to handle up to 100W input.