What is the 'awrah for men? Should a man lower his gaze on seeing another shirtless man?

Should I lower my gaze whenever I see a shirtless man, and is it **Haram** for me to look at him? And is it Haram for me to look at the knees of others because knees are supposed to be covered? I looked on the internet for an answer, but many of them had said thing which were confusing.