In Harry potter how does sirius black get his head in the griffindor fireplace when its not on the flu network

The Gryffindor fireplace is on the network, I don't remember any moment in the books when they state the contrary. It's just in the fifth book all fireplaces at Hogwarts are tracked at Umbridges order. Except for her own, which Harry later uses to talk to Sirius and Lupin.