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Chapter 3. IPv6 Addressing > Structure of a Global Unicast Address

Structure of a Global Unicast Address

The following sections examine the basic structure of a global unicast address. Global unicast addresses are also known as aggregatable global unicast addresses. These are addresses that are globally routable and reachable on the IPv6 Internet. They are equivalent to public IPv4 addresses.

Figure 3-4 shows the structure of a global unicast address for a typical site.


Figure 3-4. Structure of a Global Unicast Address for a Typical Site

Global Routing Prefix

The global routing prefix is the prefix or network portion of the address assigned by the provider, such as an ISP, to a customer or site. Although the IESG and IAB no longer recommend specific prefix lengths for different-size networks, it is still common for RIRs such as ARIN to have the policy for end sites to use a 48-bit prefix (/48). Figure 3-4 shows a typical /48 global routing prefix.


RFC 4291 does not specify the size of the Subnet ID. The 16-bit Subnet ID in Figure 3-4 results from a site receiving a /48 global routing prefix. With a 128-bit Interface ID, this leaves 16 bits for the Subnet ID. See RFC 3587, IPv6 Global Unicast Address Format, for more information.

Subnet ID

A big difference between IPv4 and IPv6 addresses is the location of the subnet portion of the address. In IPv4, bits are borrowed from the host portion of the address to create subnets. With IPv6, the Subnet ID is a separate field and is not part of the host portion of the address, known as the Interface ID in IPv6.

As shown in Figure 3-4, the IPv6 address has a 16-bit Subnet ID. This allows 65,536 individual subnets. Just in case you’re wondering—yes, you can use the all 0s and the all 1s subnets. Subnetting is discussed in the next section.

Interface ID

The Interface ID uniquely identifies the interface on the subnet. As shown earlier, the 64-bit Interface ID allows 18,446,744,073,709,551,616 (18 quintillion) addresses for each subnet! The term Interface ID is used rather than Host ID because, as mentioned previously, a single host can have multiple interfaces, each having one or more IPv6 addresses.

Another important difference between the IPv6 and IPv4 addresses is that the all-0s and all-1s addresses are legal IPv6 interface addresses. An IPv6 Interface ID can contain all 0s or all 1s. In IPv4, all 0s in the host portion of the address are reserved for the network or subnet address. All 1s in the host portion of an IPv4 address indicate a broadcast address. Remember, there is no broadcast address in IPv6.

3-1-4 Rule

IPv6 global unicast addresses can look complicated, and it can be difficult to recognize all the parts. A simple technique that I created to quickly break it down is my 3-1-4 rule, as shown in Figure 3-5. Each number refers to the number of hextets, or 16-bit segments, of that portion of the address. Perhaps an easy way to remember these numbers is to call it the pi rule (pi = 3.14).


Figure 3-5. Global Unicast Addresses and the 3-1-4 Rule

3: This indicates the three hextets, or 48 bits, of the Global Routing Prefix.

1: This indicates the one hextet, or 16 bits, of the Subnet ID.

4: This indicates the four hextets, or 64 bits, of the Interface ID.


This technique is useful whenever the global unicast address has a /48 global routing prefix and a 64-bit Interface ID, which is a common prefix allocation. As discussed later in this chapter and later in the book, Global Routing Prefixes and Interface IDs do not necessarily have to be 48 bits and 64 bits, respectively.

Table 3-8 shows several examples of /48 global unicast addresses using the 3-1-4 technique. Although the double colon compresses the notation of the address, it can sometimes make it more difficult to recognize the three parts of the address. Sometimes it can be easier to start from the Interface ID, or from both ends toward the middle Subnet ID.

Table 3-8. Examples of /48 Global Unicast Addresses with the 3-1-4 Technique


Notice that both of the following addresses are legal interface (host) addresses in IPv6:

All 0s address: 2001:DB8:ABC:: or 2001:0DB8:0ABC:0000:0000:0000:0000:0000

All 1s address: 2001:DB8::FFFF:FFFF:FFFF:FFFF:FFFF or 2001:DB8:0000:FFFF:FFFF:FFFF:FFFF:FFFF

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