From the November issue: 90210. 06831. 75201. This month, The Real Deal looks at the nation’s most exclusive zip codes — neighborhoods filled with spacious beachside mansions, panoramic mountain views and historic brick townhomes — to see how they’ve fared during the recent housing downturn. For the most part, these exclusive neighborhoods remain somewhat insulated from the turmoil of the devastated real estate markets around them. But as homes take longer to sell, even in America’s toniest areas, the first cracks in their real estate markets are beginning to show.