How to generate an enable signal when a bus value is in a very small range?
This function is in general called a decoder. 

You are correct that for this particular instance, the equivalent function is achieved by ORing the 21 highest-order bits.

A more typical decoder chip will have several outputs, each one going high for a different range of the input bits. Also, 21-bit-wider are not AFAIK commonly available as a single chip, but could be constructed by combining a few smaller decoders.