Blood pressure should be measured several times to get a good impression because it can vary during the day. So, one measurement is not good enough to conclude that it is too high. Also a lot of people have higher blood pressure during doctor's visit that normalizes again when they are home.
I would say 171 is not dangerous for a short while but if you have this blood pressure for prolonged periods it could cause damage. In your case, I would take more measurements and then decide (with doctor) if action is needed.
Your systolic number is way to high. To put it in perspective, there should never be a difference of more than 60 in your systolic and diastolic bp as a rule. Have you been seen by your doctor lately? You should be getting your bp checked by a physician as this is a dangerous level.