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 220.127.116.11 and the last address is 18.104.22.168, 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 22.214.171.124/26.