Block from U+2460 to U+24FF. This block was introduced in Unicode version 1.1 (1993). It contains 160 codepoints.
The Wikipedia provides the following information on block Enclosed Alphanumerics:
Enclosed alphanumerics is a Unicode block of typographical symbols of an alphanumeric within a circle, a bracket or other not-closed enclosure, or ending in a full stop. There is another block for these characters (U+1F100—U+1F1FF), encoded in the Supplementary Multilingual Plane, which contains the set of Regional Indicator Symbols as of Unicode 6.0.