Healthy BMIs and ideal weights are usually presented as a whole range of numbers. So depending on your frame and whether you intend to build muscle, 183 at 6 feet sounds perfectly reasonable.

I'm 5'11" and am aiming for 161, for the record. That's a totally arbitrary number I got by finding the average of various suggested weights for someone of my height.

EDIT: I ignored your other question not by negligence but by ignorance. I'm sure someone smarter than me can help figure out how to keep muscle while losing fat.

I actually don't put as much significance on BMI as I do BF%. There are several reasons for this. To me, falling within healthy or normal body fat % is more of an indicator than simply height vs weight. Because in that respect, a 6' tall athlete with an 8-pack could technically weigh more, and be considered "abnormal" according to BMI. Whereas if you look at body fat % you'd have a better indicator. I honestly wouldn't focus on BMI as a strict guideline. I assume your immediate goal is weightloss or "cutting" so I suggest aiming for a BF% for average males first. Then depending on your stretch goals, go beyond that to fitness etc.

In terms of retaining lean muscle mass, the key there is meeting your protein macro. For optimal muscle retention for males, you want to aim for 1-1.5g of protein per pound of weight. It is also suggested that for exercise you focus on strength training, heavy lifting. I should say though that eating at a caloric deficit (which I assume you will be) does no favors in terms of "building muscle", I'm sure you have read up about "newbie gains" or can easily look up the issues of cutting while building muscle. Here is an informative article: https://www.bodybuilding.com/fun/topicoftheweek88.htm

I'm not male so of course I cannot definitively say anything from experience but I have done a lot of research regarding retaining lean muscle mass, as the way of eating I follow (Keto) burns fat as fuel instead of glucose and a common concern for keto'ers is trying not to lose muscle while losing weight.

Hope this helps.

Edit: here is a good military body fat % calculator http://www.fat2fittools.com/tools/mbf/