Prefix Length Notation
In IPv4, the prefix (or network portion) of the address can be identified by a dotted-decimal netmask, commonly referred to as a subnet mask. For example, 255.255.255.0 indicates that the network portion, or prefix length, of the IPv4 address is the leftmost 24 bits. The 255.255.255.0 dotted-decimal netmask can also be written in CIDR notation as /24, indicating the 24 bits in the prefix.
IPv6 address prefixes can be represented much the same way that IPv4 address prefixes are written in CIDR notation. An IPv6 address prefix (the network portion of the address) is represented using the following format:
ipv6-address/prefix-length
The prefix-length is a decimal value indicating the number of leftmost contiguous bits of the address. It identifies the prefix (that is, the network portion) of the address. It is also used with unicast addresses to separate the prefix portion of the address from the Interface ID. Remember from Chapter 2 that the Interface ID is the equivalent to the host portion of an IPv4 address.
Let’s look at an example using the address 2001:db8:aaaa:1111::100/64. The longest preferred form in Figure 4-2 illustrates how the /64 prefix length identifies the prefix, or network portion, of the address. The /64 prefix length leaves another 64 bits, which is the Interface ID portion of the address.
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Figure 4-2 IPv6 Prefix and Prefix Length
In IPv6, just as in IPv4, the number of devices you can have on a network depends on the prefix length. However, due to the 128-bit length of an IPv6 address, there is no need to conserve address space as is needed with IPv4 public addresses.
In Figure 4-2, notice that the /64 prefix length results in an Interface ID of 64 bits. As we will discuss further in Chapter 5, this is a common prefix length for most end-user networks. A /64 prefix length gives us 18 quintillion devices on a single network (or subnet, if you prefer)!
Figure 4-3 shows several prefix length examples: /32, /48, /52, /56, /60, and /64. Notice that all of these examples fall on a nibble boundary, a multiple of 4 bits. Prefix lengths do not necessarily have to fall on a nibble boundary, although in most cases they do. Prefix lengths can also fall within a nibble—for example, /61, /62, or /63. We will discuss the prefix lengths, including within the nibble, more in Chapter 5 when we discuss the global unicast address, prefix allocation, and subnetting.
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Figure 4-3 IPv6 Prefix Length Examples