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 = 2**^{32} − n → n = 32 − log_{2}N

to find the prefix length. For example, if the first address is **17.24.12.64** and the last address is **17.24.12.127**, then the number of addresses in the block is **64**. We can find the prefix length as

**n = 32 − log**_{2}N = 32 − log_{2}64 =** 26**

The block is then **72.24.12.64/26**.