Speaking for the United States, there are quite a few areas that are 100% white. The largest neighborhoods being:
City, State, ZIP Code - Population
1. Jessup, PA, 18434 - 4,107
2. Kinsman, OH, 44428 - 3,396
3. Naples, ME, 04055 - 3,226
4. Alum Creek, WV, 25003 - 3,139
5. Shelbiana, KY, 41562 - 2,853
Reference:
United States Census ZIP Code Demographics