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IPv6 Address Representation and Address Types

Chapter Description

In this chapter from IPv6 Fundamentals: A Straightforward Approach to Understanding IPv6, 2nd Edition, author Rick Graziani examines all the different types of IPv6 addresses in the unicast, multicast, and anycast categories.


This chapter explains the basics of IPv6 addressing. The preferred format of an IPv6 128-bit address is written as eight 16-bit segments (hextets), separated by colons. The notation of the address can be reduced by omitting leading 0s and by using the double colon to replace contiguous hextets of 0s.

The IPv6 address space is extremely large. IPv6, with its 128-bit address space, provides for 340 undecillion addresses. Currently, only one-eighth of this space has been allocated for global unicast addresses, and a very small portion has been allocated for other unicast and multicast addresses.

This chapter introduces the three types of IPv6 addresses: unicast, multicast, and anycast. The following is a brief description of each of the addresses as discussed in this chapter:

  • Unicast addresses: A unicast address uniquely identifies an interface on an IPv6 device. A source IPv6 address must be a unicast address. There are several types of unicast addresses:

    • Global unicast addresses (GUAs): Global unicast addresses are also known as an aggregatable global unicast address. These addresses are globally routable and reachable on the IPv6 Internet. They are equivalent to public IPv4 addresses. The current GUA address assignment from IANA begins with the binary value 001 or the prefix 2000::/3.

    • Link-local addresses: A link-local unicast address (fe80::/10) is a unicast address that is confined to a single link. The uniqueness of this address only has to be assured on that link because these packets are not routable off the link. An IPv6-enabled device must have a link-local address. Link-local unicast addresses are usually automatically created but can also be manually configured.

    • Loopback addresses: A loopback address is an all-0s address except for the last bit, which is set to 1. It is equivalent to the IPv4 loopback address,

    • Unspecified addresses: An unspecified unicast address is an all-0s address. It cannot be assigned to an interface. An unspecified unicast address is used as a source address to indicate the absence of an address.

    • Unique local addresses: A unique local address (fc00::/7) is similar to the RFC 1918 private address space in IPv4. Unique local addresses should not be routable in the global Internet. They are to be used in more limited areas, such as within a site, or routed between a limited number of sites.

    • IPv4 embedded addresses: IPv6 addresses aid in the transition from IPv4 to IPv6. An IPv4 embedded address carries an IPv4 address in the low-order 32 bits. This type of address is used to represent an IPv4 address inside an IPv6 address. IPv4-mapped IPv6 addresses are the current type of IPv4 embedded addresses, with IPv4-compatible IPv6 addresses having been deprecated.

  • Multicast addresses: Multicast is a technique used in which a device sends a single packet to multiple destinations simultaneously. This chapter introduces two types of multicast addresses:

    • Well-known multicast addresses: These multicast addresses are reserved for predefined groups of devices, such as all-IPv6 nodes and all-IPv6 routers multicast groups.

    • Solicited-node addresses: Every unicast address assigned to an interface also has a special multicast address known as a solicited-node multicast address. These multicast addresses are automatically created using a special mapping by prepending the solicited-node multicast prefix ff02:0:0:0:0:1:ff00::/104 to the last 24 bits of the unicast address. IPv6’s solicited-node multicast address provides a way to reach every device on the link without all those devices needing to process the contents of the packet.

    • Anycast addresses: An IPv6 anycast address is an address that can be assigned to more than one interface (typically different devices). In other words, multiple devices can have the same anycast address. A packet sent to an anycast address is routed to the “nearest” interface having that address, according to the router’s routing table.

There is no broadcast address in IPv6. Instead, IPv6 uses multicast addresses such as the solicited-node multicast and all-IPv6 devices multicast.

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