Block from U+1F100 to U+1F1FF. This block was introduced in Unicode version 5.2 (2009). It contains 173 codepoints.
The Wikipedia provides the following information on block Enclosed Alphanumeric Supplement:
Enclosed Alphanumeric Supplement is a Unicode block consisting mostly of Latin alphabet characters enclosed in circles, ovals or boxes, used for a variety of purposes. It is encoded in the range U+1F100–U+1F1FF in the Supplementary Multilingual Plane.
It also includes the regional indicator symbols to be used for emoji country flag support.