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Introduction to IPv6 Addresses

IPv6 uses a 128-bit addressing model compared with the 32-bit addresses used for IPv4. In addition to being larger, IPv6 addresses differ from IPv4 addresses in several ways:

  • Notation

  • Prefixes

  • Address types

These differences give IPv6 addressing greater simplicity and scalability than IPv4 addressing gives.

IPv6 Notation

IPv6 addresses are 128 bits long (expressed as 32 hexadecimal numbers) and consist of eight colon-delimited sections. Each section contains 2 bytes, and each byte is expressed as a hexadecimal number from 0 through FF.

An IPv6 address looks like this:


By omitting the leading zeroes from each section or substituting contiguous sections that contain zeroes with a double colon, you can write the example address as:

2001:db8:0:0:0:800:200c:7334 or 2001:db8::800:200c:7334

You can use the double-colon delimiter only once within a single IPv6 address. For example, you cannot express the IPv6 address 2001:db8:0000:0000:ea34:0000:71ff:fe01 as 2001:db8::ea34::71ff:fe0.

IPv6 Prefixes

An IPv6 address prefix represents a block of address space or a network. The prefix is a combination of an IPv6 prefix (address) and a prefix length. It takes the form ipv6-prefix/prefix-length.

IPv6 addresses can be broken into prefixes of varying length. The prefix length is a decimal value that specifies the number of the leftmost bits in the address that make up the prefix. The prefix length follows a forward slash and, in most cases, identifies the portion of the address owned by an organization. All remaining bits (up to the right-most bit) represent individual nodes or interfaces.

For example, 2001:db8:0000:0000:250:af:34ff:fe26/64 has a prefix length of 64.

The first 64 bits of this address (2001:db8:0000:0000) are the prefix. The rest (250:af:34ff:fe26) identify the interface.

IPv6 Address Types

There are three major categories of IPv6 addresses:

  • Unicast—For a single interface.

  • Multicast—For a set of interfaces on the same physical medium. A packet is sent to all interfaces associated with the address.

  • Anycast—For a set of interfaces on different physical media. A packet is sent to only one of the interfaces associated with this address, not to all the interfaces.

Unicast Addresses

A unicast address identifies a single interface. When a network device sends a packet to a unicast address, the packet goes only to the specific interface identified by that address. Unicast addresses support a global address scope and two types of local address scopes.

A unicast address consists of n bits for the prefix, and 128 – n bits for the interface ID.

In the IPv6 implementation for a subscriber access network, the following types of unicast addresses can be used:

  • Global unicast address—A unique IPv6 address assigned to a host interface. These addresses have a global scope and essentially the same purposes as IPv4 public addresses. Global unicast addresses are routable on the Internet.

  • Link-local IPv6 address—An IPv6 address that allows communication between neighboring hosts that reside on the same link. Link-local addresses have a local scope, and cannot be used outside the link. They always have the prefix FE80::/10.

  • Loopback IPv6 address—An IPv6 address used on a loopback interfaces. The IPv6 loopback address is 0:0:0:0:0:0:0:1, which can be notated as ::1/128.

  • Unspecified address—An IPv6 unspecified address is 0:0:0:0:0:0:0:0, which can be notated as ::/128.

Multicast Addresses

A multicast address identifies a set of interfaces that typically belong to different nodes. When a network device sends a packet to a multicast address, the device broadcasts the packet to all interfaces identified by that address. IPv6 does not support broadcast addresses, but instead uses multicast addresses in this role.

Multicast addresses support 16 different types of address scope, including node, link, site, organization, and global scope. A 4-bit field in the prefix identifies the address scope.

The following types of multicast addresses can be used in an IPv6 subscriber access network:

  • Solicited-node multicast address—Neighbor Solicitation (NS) messages are sent to this address.

  • All-nodes multicast address—Router Advertisement (RA) messages are sent to this address.

  • All-routers multicast address—Router Solicitation (RS) messages are sent to this address.

Multicast addresses use the prefix FF00::/8.

Anycast Addresses

An anycast address identifies a set of interfaces that typically belong to different nodes. Anycast addresses are similar to multicast addresses, except that packets are sent only to one interface, not to all interfaces. The routing protocol used in the network usually determines which interface is physically closest within the set of anycast addresses and routes the packet along the shortest path to its destination.

There is no difference between anycast addresses and unicast addresses except for the subnet-router address. For an anycast subnet-router address, the low-order bits, typically 64 or more, are zero. Anycast addresses are taken from the unicast address space.

For more information about anycast addresses, see RFC 2526, Reserved IPv6 Subnet Anycast Addresses.