If the first and the last addresses are known, the block is fully defined. We can first find the number of addresses in the block. We can then use the relation
N = 232 − n → n = 32 − log2N
to find the prefix length. For example, if the first address is 126.96.36.199 and the last address is 188.8.131.52, then the number of addresses in the block is 64. We can find the prefix length as
n = 32 − log2N = 32 − log264 = 26
The block is then 184.108.40.206/26.