IPv6 Address Types: Global, Link-Local, Multicast Explained

IPv6 addresses work differently than IPv4 in how they deliver packets. Understanding these differences matters when you're debugging network issues or designing systems.

Three Delivery Methods#

IPv6 uses three fundamental delivery models:

• Unicast: One sender to one receiver. The standard way to communicate with a specific host.
• Multicast: One sender to multiple receivers. Efficient for group communication.
• Anycast: One sender to the nearest receiver in a group. Used for load balancing and redundancy.

Unicast Addresses#

Every unicast address identifies exactly one network interface. Send a packet to that address, and exactly one host receives it.

Global Unicast Addresses (2000::/3)#

These are your public IPv6 addresses, routable across the entire Internet. If an address starts with 2 or 3, it's a global unicast address.

2001:0db8:85a3:0000:0000:8a2e:0370:7334
│         │         │              │
│         │         │              └─ Interface ID (64 bits)
│         │         └──────────────── Subnet ID (16 bits)
│         └────────────────────────── Site Prefix (48 bits)
└──────────────────────────────────── Global Routing Prefix

The typical structure splits a /48 allocation like this:

Real providers typically assign a /48 to sites, a /56 to small businesses, or a /64 to residential users. Each /64 subnet contains 18 quintillion addresses, which is why you subnet on /64 boundaries.

Link-Local Addresses (fe80::/10)#

Every IPv6 interface automatically generates a link-local address when you enable IPv6. These addresses work only on the local network segment and never get routed.

fe80::1
fe80::a1b2:c3d4:e5f6:7890

Link-local addresses are mandatory for IPv6 to function. Neighbor Discovery, Router Solicitation, and other core protocols require them. Your interface might not have a global address, but it will always have a link-local address.

Zone IDs: Specifying the Interface#

Since fe80::1 could exist on every interface, you need to specify which interface you mean:

ping6 fe80::1%eth0      # Linux
ping6 fe80::1%en0       # macOS
ping fe80::1%12         # Windows (use interface index)

The % symbol precedes the zone ID. On Linux and macOS, use the interface name. On Windows, use the interface index from netsh interface ipv6 show interface.

Unique Local Addresses (fc00::/7)#

Think of these as IPv6's version of RFC 1918 private addresses. They're not globally routable, making them suitable for internal networks.

In practice, you'll see fd00::/8 addresses because the fc00::/8 range requires central allocation that was never implemented. Generate the 40-bit global ID randomly:

fd 3a:c7b1:29f4 :0001:0000:0000:0000:0001
│  └──────────┘  │    └──────────────────┘
│       │        │             │
│       │        │             └─ Interface ID (64 bits)
│       │        └─────────────── Subnet ID (16 bits)
│       └──────────────────────── Random Global ID (40 bits)
└──────────────────────────────── ULA prefix (fd = locally assigned)

When you generate that 40-bit identifier randomly, you minimize collision risk if two networks merge later. Don't just use fd00::1 everywhere.

Special Addresses#

Two addresses have unique purposes:

Loopback (::1): Send packets to yourself. The IPv6 equivalent of 127.0.0.1. Traffic to ::1 never leaves the host.

Unspecified (::): Represents the absence of an address. Hosts use :: as a source address during DHCPv6 or when they haven't configured an address yet. You can't send packets to ::.

Multicast Addresses (ff00::/8)#

Every address starting with ff is multicast. The packet goes to every interface that joined that multicast group.

Address Structure#

ff 0 2 : 0000:0000:0000:0000:0001
│  │ │   └──────────────────────┘
│  │ │              │
│  │ │              └─ Group ID (112 bits)
│  │ └──────────────── Scope (4 bits)
│  └─────────────────── Flags (4 bits)
└────────────────────── Multicast prefix

The scope field determines how far the multicast packet travels:

Essential Multicast Groups#

Several multicast addresses serve critical functions:

Solicited-Node Multicast#

This mechanism makes neighbor discovery efficient. Instead of asking every host "who has this IP?" (like ARP), IPv6 asks a small multicast group.

Every unicast address automatically generates a corresponding solicited-node multicast address:

Unicast:        2001:db8:1234:5678::abcd:ef12:3456
                                      └────┬─────┘
                                           │
                                     (last 24 bits)
                                           │
Solicited-Node: ff02::1:ff12:3456 ────────┘

When a host needs to resolve 2001:db8:1234:5678::abcd:ef12:3456, it sends a Neighbor Solicitation to ff02::1:ff12:3456. Only hosts with addresses ending in 12:3456 process the request. This dramatically reduces multicast overhead compared to broadcasting to every host.

Anycast Addresses#

Anycast addresses look identical to unicast addresses—there's no special prefix. The difference is configuration: you assign the same address to multiple interfaces, and routing delivers packets to the topologically nearest one.

Common Use Cases#

DNS Root Servers: All 13 root server letters (a.root-servers.net through m.root-servers.net) use anycast. The same IP addresses exist at hundreds of locations worldwide. Your query reaches whichever root server is closest.

CDN Edge Servers: Content delivery networks use anycast to route users to nearby cache servers. Same address, different geographic locations.

6to4 Relay Routers: The address 192.88.99.1 (IPv4) and 2002:c058:6301:: (IPv6) are anycast addresses for 6to4 relays.

Subnet-Router Anycast#

Every subnet automatically has a reserved anycast address with the interface identifier set to zero:

Subnet:         2001:db8:1234:5678::/64
Subnet-Router:  2001:db8:1234:5678::

Routers on that subnet should respond to this address. In practice, it's rarely used.

Quick Reference: Identifying Address Types#

Look at the first few characters to identify any IPv6 address:

Everything else is either reserved or not yet assigned.

Checking Your Addresses#

See what addresses your system has configured:

Display IPv6 addresses configured on your system:

Linux:

ip -6 addr show

This command lists all IPv6 addresses assigned to each network interface.

macOS:

ifconfig | grep inet6

This filters interface configuration to show only IPv6 addresses.

Windows:

netsh interface ipv6 show addresses

This displays all configured IPv6 addresses with interface details.

You'll typically see:

• Multiple link-local addresses (fe80::), one per interface
• One or more global unicast addresses if you have IPv6 connectivity
• The loopback address ::1 on the loopback interface
• Several multicast group memberships (usually not displayed by default)

Testing Multicast#

Send an ICMPv6 echo request to all nodes on your local link:

# Replace eth0 with your interface name
ping6 ff02::1%eth0

This sends a multicast ping to all IPv6-enabled devices on the local network segment. You'll get replies from every device, which is useful for discovery but can be noisy on large networks.

Check if routers are present:

ping6 ff02::2%eth0

This targets the all-routers multicast group. Only routers should respond to this address.

Practical Implications#

Different address types affect how you design networks and troubleshoot issues:

Security policies: Filter global unicast at your firewall, but link-local must pass through for neighbor discovery. Blocking fe80::/10 breaks IPv6 entirely.

Application binding: Servers that bind to :: listen on all addresses (like 0.0.0.0 in IPv4). Binding to ::1 restricts to localhost only.

Routing tables: Link-local addresses appear as next-hops in routing tables, which confuses people used to IPv4. This is normal—routers use link-local addresses for stability since they never change.

DNS: Only publish global unicast addresses in DNS. Never publish link-local or unique local addresses in public DNS zones.

Understanding these address types isn't academic—it's necessary for configuring firewalls, debugging connectivity, and designing networks that actually work.

Related Articles#

• IPv6 Fundamentals - Understanding the basics of IPv6 addressing and why it matters
• IPv6 Subnetting Guide - Learn how to efficiently divide your IPv6 address space
• IPv6 Security Best Practices - Secure your IPv6 network against common threats and vulnerabilities