A homemade bomb is a terrorist’s favorite device. This is one of the lessons for 45 city cops, firefighters and emergency responders training today for a potential catastrophe.

Trainers from the state’s Division of Homeland Security and Emergency Services are at the FDNY Fire Academy today on Randall’s Island, teaching city responders about improvised explosive devices. In a bombing’s aftermath, responders — from firefighters at the scene to doctors at a hospital — should be able to perform patient triage and evacuate bystanders.

“Terrorists look for IEDs as an instrument of terror,” Cliff Parker, assistant director of training at the State’s Office of Counter Terrorism, told Metro.

Around the holidays, police typically boost patrols, and the FDNY plans to have a few extra ambulances ready for large crowds like Times Square on New Year’s Eve — just in case.

Would-be New York terrorists eye bombs, Parker said.