How to still cache across sites

Seems like if you want to retrieve and/or generate a N-bit id that it would take more than N images. For instance, if you had a 32bit id and loaded 32 images, then afterwards you have all 32 in cache and attempts to read it afterwards would give id of 0xffffffff . You might want 2*M*N images at least. I think that I can make a probabilistic scheme to retrieve an id without poisoning the cache too much. I might also want to include some kind of FEC to the scheme