By the 1980s, the global Internet was growing quickly enough that it became obvious more IP address space would be needed. While several schemes to resolve this problem were proposed, only two are widely deployed today: IPv6 and Network Address Translation (NAT).

Note
IPv6 is completely different than IPv4, but we are only concerned with the changes in addressing here. Other changes between IPv4 and IPv6 will be considered in Chapter 14.

The term IP is used when both IPv4 and IPv6 are intended throughout this book.

IPv6 was initially accepted as a draft standard by the Internet Engineering Task Force (IETF) in December 1998, and the first IPv6 addresses were allocated in July of 1999. IPv4 and IPv6 will likely co-exist in most networks for a long time.

In designing IPv6, the IETF quadrupled the address space. Rather than 32 bits divided into four one octet sections, the IPv6 address is 128 bits divided into 16 sections. Each section, sometimes called a quartet, represents two octets of the address using four hexadecimal digits. Figure 2-9 illustrates an IPv6 address.

Figure 2-9 An IPv6 Address

IPv6 addresses include a prefix length to differentiate between the prefix and subnet addresses—just like IPv4—but the maximum prefix length is now /128 rather than /32. Longer addresses are more difficult to work with, but IPv6 addressing is also simplified in some ways:

•Individual hosts always receive a /64 address, and links between network devices normally receive a /128 address. Prefix lengths between /64 and /128 are extremely uncommon.

•The shortest prefix most networks will be allocated will be a /48. Larger companies and service providers may have access to address space with a prefix length as short as a /29, but most of the addresses you will be working with daily will have prefix lengths longer than /48.

•Any single long string of 0s can be replaced with a double colon or :: (you can use the :: only once in an address).

•All leading 0s are omitted.

These simplifications mean you will mostly work with addresses with prefix lengths between a /48 and a /64, or about 16 possible lengths. Much like IPv4 addresses, the simplest way to work with IPv6 prefix lengths—if you insist on working with IPv6 addresses by hand—is by using skips, as shown in Table 2-3. You can find the study guide at https://www.acedexam.com/300-430-enwlsi-implementing-cisco-enterprise-wireless-networks/

Table 2-3 IPv6 Address Skips

For instance, for 2001:db8:3e8::/48 prefix:
•You can create two /49 subnets, 2001:db8:3e8::/49 and 2001:db8:3e8:8000::/49.
•You can create four /50 subnets, 2001:db8:3e8::/50, 2001:db8:3e8:4000::/50, 2001:db8:3e8:8000::/50, and 2001:db8:3e8:c000::/50.
•2001:db8:3e8:500::/54 is not a valid prefix; you count by fours in the second digit for /54s, and 5 is not a multiple of 4.
Just like in IPv4, the first and last address of the subnet are broadcast addresses.

Three further points:
•After working with IPv6 addresses for a while, you will probably recognize common prefix lengths and where their prefixes begin and end.
•Most network operators carefully plan their addressing so only a few prefix lengths are used; this simplifies becoming familiar with them and makes spotting mistakes easy.
•While working with IPv6 addresses, you should use a subnet calculator and/or cheat sheet to prevent mistakes.