Border protection grids were security systems utilized by the United Federation of Planets to protect individual member worlds. Each grid utilized specific subspace frequencies which, if known, could be used to bypass the grid. These frequencies were considered to be highly-privileged information, and as such, only select groups of people were privy to them, including Starfleet captains.
In 2258, Nero, master of the Romulan mining vessel Narada, utilized a Centaurian slug to compel Captain Christopher Pike to provide him with the subspace frequencies of the border protection grid surrounding Earth, in an effort to approach the planet unobstructed and launch an attack. (Star Trek)