IPv6 has more address types than IPv4. Where IPv4 has unicast, multicast, and broadcast (gone in IPv6), IPv6 has global unicast, link-local, unique local, multicast, anycast, and a few special addresses. Each has a specific scope and purpose, and getting the types right is foundational to IPv6 design.
This article walks through each address type, when to use which, and the operational implications. If you are designing IPv6 addressing or trying to figure out why show ipv6 interface shows so many addresses on every interface, this is the reference.
The Six Address Types
Global Unicast
Global unicast addresses are the IPv6 equivalent of IPv4 public addresses. They are globally routable and globally unique. The currently allocated range is 2000::/3 (any address starting with 2 or 3 in the first hex digit).
The structure of an enterprise global unicast address:
+- 48 bits global prefix -+- 16 bits subnet -+- 64 bits interface ID -+
| 2001:db8:1234: | abcd: | xxxx:xxxx:xxxx:xxxx |
+-------------------------+------------------+-------------------------+The 48-bit prefix is what your ISP allocates to you. The 16-bit subnet field gives you 65,536 subnets within that prefix, plenty for any enterprise. The 64-bit interface ID identifies the host on the subnet.
Examples:
2001:db8::/32 is reserved for documentation; never use this on real production networks.
Link-Local
Link-local addresses are the workhorse of IPv6 internal protocols. Every IPv6 interface has one, automatically assigned, and it is required for the interface to function. The prefix is FE80::/10 (in practice always FE80::/64 because the next 54 bits must be zero).
Properties:
- Always present on every IPv6 interface (required by the protocol)
- Never crosses a router (link-scoped)
- Used for Neighbor Discovery, Router Solicitation/Advertisement, OSPFv3 adjacencies, EIGRP for IPv6 sessions, VRRPv3 advertisements
- Format: FE80:: followed by 64 bits of interface ID (often EUI-64 derived from MAC)
- Same link-local can theoretically appear on multiple interfaces of the same router (you scope it with %interface-id syntax:
fe80::1%Gi0/0/0)
Verify with:
Router# show ipv6 interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::21A:2BFF:FE3C:4D5E
Global unicast address(es):
2001:DB8:1::1, subnet is 2001:DB8:1::/64Most routing protocols (OSPFv3, EIGRP for IPv6) form adjacencies over link-local addresses, not global unicast. The benefit: if you renumber the global prefix, the IGP keeps running uninterrupted.
Unique Local Addresses (ULAs)
ULAs are the IPv6 equivalent of IPv4 RFC 1918 private addresses. RFC 4193 defines the format. The prefix is FC00::/7, but in practice you only use FD00::/8 (the L bit is set to 1 to indicate locally-assigned).
The ULA structure includes a 40-bit Global ID that is meant to be pseudo-random:
+- FD -+- 40-bit Global ID --+- 16-bit subnet -+- 64-bit interface ID -+
| FD | xxxxxxxxxxxxxxxxxx | abcd | xxxx:xxxx:xxxx:xxxx |
+------+---------------------+-----------------+------------------------+You generate a random Global ID once for your organization (using SHA-1 of the time and MAC, or just using a random generator) and use it across all your sites. The randomness reduces collision risk if two ULA-using networks ever merge.
Use ULAs for:
- Internal management traffic that should never leak to the internet
- Communication between hosts that don't need internet reachability
- Lab environments where global addresses don't make sense
- IPv6 addressing for networks that have private IPv4 addressing today
Modern IPv6 best practice often skips ULAs entirely - use global unicast everywhere with proper firewalling, since you have abundant addresses. ULAs are a tool for specific use cases, not the default.
Multicast
IPv6 has no broadcast. Functions that used to use broadcast (ARP, etc.) now use multicast. The prefix is FF00::/8.
The multicast address format includes scope bits:
+- FF -+- Flags (4) -+- Scope (4) -+- Group ID (112) -+
| FF | flgs | scope | group identifier |
+------+-------------+-------------+-------------------+Common multicast scopes:
Well-known multicast addresses you see constantly:
The solicited-node multicast is interesting: each unicast address has a corresponding multicast address derived from the bottom 24 bits. NDP uses this to direct Neighbor Solicitations only to hosts whose interface ID matches, dramatically reducing broadcast-style flooding.
Anycast
Anycast is "one address, multiple hosts." Multiple hosts advertise the same address; the network delivers to whichever is topologically closest. It is technically just a unicast address shared by multiple hosts; nothing in the address format indicates anycast (it is up to the routing layer).
Common uses: DNS root servers (root anycast), CDN edge servers, internal services like NTP across multiple data centers. The "subnet anycast" address (interface ID all zeros within a subnet) is reserved by RFC 4291.
Special Addresses
Every Interface Has Multiple Addresses
An IPv6-enabled interface typically has:
- A link-local address (FE80::xxxx)
- One or more global unicast addresses (2xxx:xxxx::xxxx)
- Optionally a ULA (FDxx:xxxx::xxxx)
- Multicast group memberships (FF02::1 always; FF02::5 if OSPFv3; etc.)
- Solicited-node multicast for each unicast address
This is normal IPv6 behavior. Cisco's show ipv6 interface displays them all.
Design Implications
Summary
IPv6 has six main address types: global unicast, link-local, ULA, multicast, anycast, and special addresses. Every interface gets at least a link-local automatically; you assign global unicast for internet reachability and ULAs for site-internal traffic. Multicast is built into the protocol and used pervasively (NDP, RAs, OSPFv3, etc.).
Master the prefixes (2000::/3 for global, FE80::/10 for link-local, FC00::/7 for ULA, FF00::/8 for multicast) and the rest of IPv6 addressing follows. Bookmark this article alongside the IPv6 cluster pillar.
