Are Fajr & Isha the Nautical Twilights?

For those of you who don't know what it is, consider going through this page or this is good too. The main concern is with morning fajr/Dawn prayer. People say the white thread - black thread, honestly, those terms don't pretty much get into my head. So, I just understood them as simply the turn over from dark to light i.e. the moment you feel there is a change. Is my understanding right? But, practically, in this modern world I can't stand and exactly measure when is there gonna be a change. I would simply rely on the internet and check when is that twilight happening in my place. Here is my concerned page . The obvious concern is because I want to determine the exact time of ending my Suhr. I could safely pray my Fajr prayer after the nautical twilight anyways. If you've gone through the page, there is a lot of difference between the nautical and Astro twilights in morning. So, wanted the exact time until when I can enjoy my Suhr - When does Fajr start / Sahr end? Is it Astro, Nautical or Civil twilight? Also, a related question, - When does Isha start / Maghrib end? Is it Astro, Nautical or Civil twilight? (I guess it is Nautical here, but want a confirmation)