2.2. Common Prefix Length

We define the common prefix length CommonPrefixLen(S, D) of a source address S and a destination address D as the length of the longest prefix (looking at the most significant, or leftmost, bits) that the two addresses have in common, up to the length of S's prefix (i.e., the portion of the address not including the interface ID). For example, CommonPrefixLen(fe80::1, fe80::2) is 64.