Border protection grid

Template:2009spoiler Border Protection Grids were a system used in an alternate 2258 to protect some planets of the United Federation of Planets, Including Earth. They could be bypassed if an attacker knew their subspace frequencies, information to which Starfleet captains were privy.

Nero used an Centaurian slug to extract the frequencies of the Earth grid from Christopher Pike. (Star Trek)

It is unknown if all federation planets have such grid, since Nero intended to attack all federation planets, but was only interested in the frequencies of Earth's.