Hi,
This only happens when a network adapter wants to use promiscuous mode.
This means that the network adapter will not only being able to monitor the network traffic that is meant for that adapter, but to also sniff on all of the network traffic that simply passes by. As being able to look at network traffic that is not directly targeted at that network adapter is a special case, you need to have administrative rights.
Hence the popup.
There are a few other use cases however.
For example, if you try to run a virtualized vSphere instance then that nested VM has network adapters that need to be able to assign network addresses in any range, this also requires the same "promiscuous" mode.
For more info see:
and
http://www.virtuallyghetto.com/2013/11/why-is-promiscuous-mode-forged.html
--
Wil
