Maginot Line

The Maginot line was a Forerunner defense line that protected the Forerunner inner colonies. It's precise location is unknown but from records drawn from the Halo 3 terminals we can assume that Earth was not inside it and niether were most worlds the Librarian visited.

It was heavily damaged when the Flood made a large attck on it led by the rampant Forerunner AI Mendicant Bias. It was the main line of defense holding the flood back and it is assummed it ws broken before the first firing of the Halo array. It was a sphere aruund a Forerunner controlled area of space.

For a long time it can be assummed that Diadact worked in the Msaginot line frequently sending transmissions form it to the Librarian asking her to return behind it. Although it was a defense against the flood it cannot be assumed that the flood conquered all worlds that were outside of it as Earth, which we knw was outside it was presumably not infected because the Librarian was on it looking at how beautiful it was before the Array fired.

For anymore information that has not been mentioned the Terminal text from Halo 3 is very useful.

"Would you like to see the relevent data?"

http://halo.wikia.com/wiki/Terminals