In IPv4, the prefix of the address (the network portion) can be represented either by a dotted-decimal netmask (the subnet mask) or through CIDR notation. When we see 192.168.100.0 255.255.255.0 or 192.168.100.0/24, we know that the network portion of the address is the first 24 bits of the address (192.168.100) and that the last 8 bits (.0) are host bits. IPv6 address prefixes are represented in much the same way as IPv4 address prefixes are written in CIDR notation. IPv6 prefixes are represented using the following format:
The prefix-length is a decimal value showing the number of leftmost contiguous bits of the address. It identifies the prefix (the network portion) of the address. In unicast addresses, it is used to separate the prefix portion from the Interface ID. The Interface ID is equivalent to the host portion of an IPv4 address.
Looking at the address
we know that the leftmost 64 bits are the prefix (network portion) and the remaining bits are the Interface ID (host portion). See Figure 1.
Figure 1 IPv6 Prefix and Prefix Length
A /64 prefix length results in an Interface ID of 64 bits. This is a common prefix length for most end-user networks. A /64 prefix length gives us 264 or 18 quintillion devices on a single network (or subnet).
There are several more common prefix length examples, as shown in Figure 2. All of these examples fall either on a hextet boundary or on a nibble boundary (a multiple of 4 bits). Although prefix lengths do not need to fall on a nibble boundary, most usually do.
Figure 2 IPv6 Prefix Length Examples