8.3 Address Types

Introduction

IPv6 addresses are not all used in the same way. IPv6 defines several main address types that decide who receives the traffic and how routers treat the packets. Understanding these types is essential to read, design, and troubleshoot IPv6 networks.

In this chapter, you will learn what the main IPv6 address types are, what they look like, and how they are used at a basic level, without going into topics that belong to routing, neighbor discovery, or configuration details.

The Three Core Address Types

IPv6 has three fundamental address types:

1. Unicast
2. Multicast
3. Anycast

IPv4 also had unicast and multicast, and usually used broadcast. IPv6 removes broadcast and replaces that role with certain multicast addresses.

Each address type defines how many devices should receive a packet when it is sent to that address.

Table: High level comparison

Unicast Addresses

A unicast address identifies a single interface. Packets sent to a unicast address are delivered to one device interface, and that device processes the packet. Unicast addresses are used for most normal communication, for example when you open a website or connect to a server.

Unicast is a category. Inside it, IPv6 defines several important unicast types with specific roles and scopes.

Global Unicast Addresses

Global unicast addresses are the IPv6 equivalent of public IPv4 addresses. They are globally unique and routable on the public Internet. Hosts and routers use them for communication across networks that are not directly attached.

A global unicast address is usually in the range that starts with 2 or 3 in hexadecimal, for example:

• 2001:0db8:1234:5678:abcd:ef01:2345:6789
• 2607:f8b0:4009:080a:0000:0000:0000:2004

In short form:

• 2001:db8:1234:5678:abcd:ef01:2345:6789
• 2607:f8b0:4009:80a::2004

You will learn detailed structure and notation in the chapter on IPv6 address format. Here, focus on the role. Global unicast addresses:

• Identify interfaces that can be reached across the Internet, if routing and security policies allow it.
• Are assigned in blocks to organizations, which then assign subnets and host addresses internally.
• Replace the need for public IPv4 addresses, giving very large address space.

Link Local Addresses

Link local addresses are only valid on a single local network link. They allow devices to communicate with neighbors on the same network segment without any configuration from a server or administrator.

These addresses always begin with fe80 and have a prefix of /10. Example:

• fe80::1
• fe80::a00:27ff:fe4e:66a1

Key properties:

• Routers do not forward packets with a source or destination link local address.
• Every IPv6 enabled interface must have a link local address, even if it also has a global or other unicast address.
• Protocols that only need local link communication, such as many neighbor discovery operations, use link local addresses.

Unique Local Addresses

Unique local addresses, often called ULAs, are somewhat similar to private IPv4 addresses. They are meant for local communication inside one or more sites, and they are not intended to be routed on the public Internet.

ULAs:

• Start with the prefix fc00::/7.
• In practice, you usually see fdxx... because fd00::/8 is commonly used for locally assigned ULAs.

Example ULA:

• fd12:3456:789a::1

Typical uses:

• Internal networks that do not need or should not use global addresses.
• Lab networks or test environments.
• Overlays and internal routing that should be independent of Internet addressing.

The main differences from global unicast are scope and routing policy. ULAs are not advertised on the public Internet.

Other Unicast Related Concepts

You may encounter other special unicast ranges, for example addresses used for specific transition mechanisms or other advanced features. For learning purposes at the beginner level, focus on recognizing global unicast, link local, and unique local. These three cover most basic designs.

Multicast Addresses

Multicast addresses identify groups of interfaces that want to receive the same traffic. When a packet is sent to a multicast address, all members of that group can receive it. Multicast reduces unnecessary traffic compared to broadcast, because only devices that have joined the group process the packet.

IPv6 multicast addresses always begin with ff.

Examples:

• ff02::1
• ff02::2
• ff05::1:3

Multicast Scope

Every IPv6 multicast address has a scope. Scope tells how far the packet is allowed to travel. Common scopes include:

• Link local scope, traffic stays within a single link.
• Site local scope, traffic stays within a site.
• Global scope, traffic may cross the Internet.

The second hexadecimal digit in the multicast address often indicates scope. You will see ff02 frequently, which means link local multicast.

Table: Common scopes

Well Known Multicast Groups

IPv6 defines several special multicast groups that all IPv6 nodes or routers must join. These are used for essential network functions.

Some important examples:

• ff02::1 is the "all nodes" multicast group on the local link. Every IPv6 interface joins this group. Sending a packet to ff02::1 reaches all IPv6 nodes on the link.
• ff02::2 is the "all routers" multicast group on the local link. Every IPv6 router joins this group. Sending a packet to ff02::2 reaches all routers on that link.
• Various protocols, such as routing protocols, use their own multicast groups so that only interested routers receive the packets.

These multicast groups replace IPv4 broadcasts. For example, in IPv4 ARP and some discovery protocols sent packets to a broadcast address. In IPv6, similar purposes are achieved by sending packets to specific multicast groups.

Anycast Addresses

Anycast addresses represent a group of interfaces, usually on different devices and often in different locations, that share the same address. Unlike multicast, a packet sent to an anycast address is delivered to only one member of the group, typically the one that is "closest" according to the routing system.

Anycast uses normal unicast address format. There is no special prefix that marks an address as anycast. Instead, the address is configured on multiple interfaces and routers are told via routing advertisements that these interfaces all exist. The routing protocol then chooses the best path.

Typical uses for anycast:

• DNS server farms, where the same IPv6 address is configured on many DNS servers worldwide. Clients reach the nearest one from a routing perspective.
• Services that need geographic redundancy and load distribution.
• Default gateways for subnets in some designs.

Because anycast is not a new format, it is more a method of using unicast addresses. The key difference is that multiple devices share the same unicast address, and routing decides which one receives the traffic.

Contrast with multicast:

• Multicast delivers a packet to many receivers.
• Anycast delivers a packet to one receiver from a group.

Special Address: Unspecified Address

There is a special IPv6 address that is used when a device does not yet have a valid address to use. This is the unspecified address.

The unspecified address is:

• ::

All bits are zero. It is never assigned as a normal address to an interface and is never used as a destination in real traffic. Instead, a device uses it internally as a source address while it is still trying to obtain a valid address or while it performs certain checks.

You might see the unspecified address in logs, or during early stages of address configuration. It indicates "no address yet" rather than a real host.

Comparing IPv6 Address Types

To summarize how the main types differ, consider this simplified comparison.

Each type has a clear purpose. In an actual IPv6 network, a single interface often has multiple addresses of different types at the same time, for example:

• One link local address.
• One or more global unicast addresses.
• Possibly a unique local address.
• Membership in several multicast groups.

Understanding these types will help you interpret configuration outputs, packet captures, and design choices in later chapters.