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Windows Server 2012 : Configuring IPv6/IPv4 interoperability (part 3) - IPv6 address representation

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3. IPv6 addressing

In contrast to the 32-bit (4-byte) addresses used by the IPv4 protocol, IPv6 uses 128-bit (16-byte) addresses. Using addresses like this that are four times longer increases the theoretically usable address space from 232 addresses (approximately 4 billion addresses) to 2128 addresses (approximately 3.4 x 1038 addresses). The actual address space for IPv6 is smaller, however, because of the hierarchical way that IPv6 addresses are constructed. Specifically, each 128-bit IPv6 consists of two parts:

  • A 64-bit IPv6 prefix that indicates the address type, how packets containing the address should be routed, the subnet on which the interface having the address resides, or some combination of these types of information

  • A 64-bit IPv6 interface identifier that identifies the interface on the subnet

4. IPv6 address representation

IPv4 addresses are usually represented in the familiar dotted-decimal form, such as 65.55.58.201, where each number represents 8 bits of the 32-bit address. By contrast, the much longer IPv6 addresses are typically represented by dividing the 128-bit address into 16-bit (4-byte) segments. Each segment is then converted from binary format to a 4-bit hexadecimal number, and these numbers are finally separated by using colons.

For example, when expressed in binary form, the following IPv6 address spans two lines of text on this page:

0010000000000001000011011011100000111111101010010000000000000000
0000000000000000000000000000000000000000110100111001110001011010

When converted into colon hexadecimal notation, however, the address is much shorter:

2001:0DB8:3FA9:0000:0000:0000:00D3:9C5A

The preceding address can be further compressed by suppressing leading zeros as follows:

2001:DB8:3FA9:0:0:0:D3:9C5A

Even further compression can be achieved by representing contiguous blocks of hexadecimal zeros as double colons as follows:

2001:DB8:3FA9::D3:9C5A

Note

Double-colon notation

To ensure unambiguous representation, only one double colon can be used when representing an IPv6 address.

IPv6 prefixes

The first 64 bits of a 128-bit IPv6 address represent the IPv6 prefix for the address. An IPv6 prefix can be used to

  • Specify the type of the IPv6 address.

  • Define a summarized route.

  • Indicate a subnet.

For example, the IPv6 prefix portion of the IPv6 address 2001:DB8:3FA9::D3:9C5A used in the previous section is 2001:DB8:3FA9:0.

IPv6 prefixes are expressed using an <address>/<prefix_length> format that is similar to the Classless Inter-Domain Routing (CIDR) notation used on IPv4 networks. The value of <prefix_length> can vary as follows:

  • Subnets always have a prefix length of 64.

  • Summarized routes always have a prefix length less than 64.

For example, an IPv6 prefix of 2001:DB8:3FA9::/48 represents a summarized route.

IPv6 address types

IPv4 addresses can be either unicast, multicast, or broadcast addresses. IPv6 addresses by comparison can be either of the following:

  • Unicast This type of IPv6 address identifies a single interface within a region of an IPv6 network over which the address is unique.

  • Multicast This type of IPv6 address identifies zero or more interfaces on the same host or different hosts and is used for one-to-many communications with delivery to multiple interfaces.

  • Anycast This type of IPv6 address identifies multiple interfaces and is used for one-to-one-of-many communications with delivery to a single interface.

Unicast IPv6 addresses can be further categorized as either of the following:

  • Global unicast addresses

  • Link-local addresses

  • Unique local addresses

  • Special addresses

  • Transition addresses

The sections that follow go into greater detail about unicast address types. Note that there are no broadcast addresses in IPv6. Instead, multicast addresses are used when broadcast functionality is required over a portion of an IPv6 network.

Global unicast addresses

Global unicast addresses are IPv6 addresses that are globally routable and therefore are reachable on the IPv6 Internet. Global unicast addresses for IPv6 are the equivalent of public addresses for IPv4.

A global unicast address is always structured as follows:

  • The first three bits of the address are always 001 in binary format.

  • The next 45 bits represent the global routing prefix for the organization’s site. Taken together with the three predefined high-order bits, they define the 48-bit site prefix, which is used by routers on the IPv6 Internet to identify IPv6 packets that should be forwarded to the routers of the organization’s site.

  • The next 16 bits are used to identify the subnet within the site. Because 16 bits are available for defining subnets, each site can have up to 216 or 65,536 subnets.

  • The final 64 bits specify the interface on the indicated subnet within the site.

Note

Interfaces vs. nodes

IPv6 unicast addresses always indicate interfaces (not nodes) on an IPv6 network.

Link-local addresses are IPv6 addresses that are used whenever a node needs to communicate with a neighbor (another node on the same link). For example, if a site has no routers, and therefore only one subnet, all network communications between hosts can take place using link-local addresses.

On Windows platforms, IPv6 link-local addresses are always configured automatically on all interfaces even if no other unicast IPv6 addresses are configured. The IPv4 equivalent to these addresses are IPv4 link-local addresses, which are addresses in the range 169.254.0.0/16 that are dynamically configured on interfaces when no Dynamic Host Configuration Protocol (DHCP) server is available. On Windows platforms, IPv4 link-local addresses are assigned using Automatic Private IP Addressing (APIPA).

A link-local address is always structured as follows:

  • The first 64 bits are always 11111110 10000000 00000000 00000000 in binary format. This means that a link-local address always begins with FE80 and has a prefix identifier of FE80::/64.

  • The final 64 bits specify the interface on the local link.

Note

Link-local addresses and routers

IPv6 routers will never forward packets addressed to link-local addresses beyond the local link.

Unique local addresses

Unique local addresses are IPv6 addresses that are private to an organization in the same way that private addresses—such as 10.x.x.x, 192.168.x.x, or 172.16.0.0 - 172.31.255.255—can be used on an IPv4 network. Unique local addresses, therefore, are not routable on the IPv6 Internet in the same way that an address like 10.20.100.55 is not routable on the IPv4 Internet.

A unique local address is always structured as follows:

  • The first 8 bits are always 11111101 in binary format. This means that a unique local address always begins with FD and has a prefix identifier of FD00::/8.

  • The next 40 bits represent the global identifier, which represents a specific site within the organization. This global identifier is randomly generated.

  • The next 16 bits are used to identify the subnet within the site. Because 16 bits are available for defining subnets, each site can have up to 216 or 65,536 subnets.

  • The final 64 bits specify the interface on the indicated subnet within the site.

Special addresses

The following two addresses have special significance in IPv6:

  • The address 0:0:0:0:0:0:0:0, which is commonly represented as a double colon (::), indicates the absence of an IPv6 address. The IPv4 equivalent to this address is 0.0.0.0.

  • The loopback address 0:0:0:0:0:0:0:1, which is commonly represented as ::1, is assigned to the loopback interface on a node. The loopback address is used whenever a node needs to send a packet to itself. The IPv4 equivalent to this address is 127.0.0.1.

Transition addresses

Transition addresses are IPv6 addresses used by IPv6 transition technologies such as ISATAP, Teredo, or 6to4. Transition addresses enable the coexistence of IPv4 and IPv6 hosts on the same network. IPv6 transition technologies are described in more detail later in this lesson.

Multicast addresses

Multicasting on IPv6 networks works essentially the same way that it does on IPv4 networks. An IPv6 multicast address always begins with 11111111 or FF and includes additional structure that identifies the scope of the address and the multicast group to which the interface belongs. IPv6 multicast addresses, therefore, are always of the form FF00::/8. For comparison, IPv4 multicast addresses are always of the form 224.0.0.0/4.

As indicated earlier in this lesson, IPv6 does not have broadcast addresses and instead uses certain multicast addresses whenever some form of broadcast functionality is required. Examples of this usage include the following multicast addresses:

  • FF01::1 This address is an all-nodes multicast address that has interface-local scope.

  • FF02::1 This address is an all-nodes multicast address that has link-local scope.

  • FF01::2 This address is an all-routers multicast address that has interface-local scope.

  • FF02::2 This address is an all-routers multicast address that has link-local scope.

  • FF05::2 This address is an all-routers multicast address that has site-local scope.

For example, the equivalent of IPv6 address FF02::1 on an IPv4 network is 255.255.255.255.

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