On the project voldemort design page:
http://project-voldemort.com/design.php
It is stated that the hash ring covers the interval [0, 2^31-1].
Now, the interval [0, 2^31-1] represents 2^31 total numbers, and the largest number 2^31-1 is just 31 bits all set to 1. (To convince yourself of this, consider 2^3-1. 2^3=8 and is 0x1000. 2^3-1=7 and is 0x111).
Thus, if a normal 32-bit address word is used to store the value, you have 1 bit free.
Thus, why is 2^31-1 the upper limit? Is that extra bit used for some kind of system bookkeeping?
(e.g. 1 extra bit would provide room for safely adding two valid hash addresses without overflow).
And finally, is this choice specific to voldemort, or is it seen in other consistent hashing schemes?