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IPv6 gives every interface three potential address types. In practice you'll configure two — a global unicast and rely on auto-generated link-local. Understand all three and you'll know what you're looking at in a `show ipv6 interface` output.
GUA is what you put on interfaces that need internet reachability. Prefix `2000::/3`, assigned by your ISP or RIR. The structure is `2001:db8:1234:5678::/64` where the first 48 bits are the ISP's global routing prefix, the next 16 are your subnet ID, and the final 64 are interface IDs.
Router1(config)# ipv6 unicast-routingRouter1(config)# interface gigabitEthernet0/0Router1(config-if)# ipv6 address 2001:db8:acad:1::1/64Router1(config-if)# no shutdown
`ipv6 unicast-routing` enables IPv6 routing globally — without it, the router won't forward IPv6 packets even if interfaces have addresses. This is the most common gotcha: interfaces configured, no traffic passing, because routing is disabled.
Every IPv6 interface auto-generates a link-local address starting with `fe80::`. It's used for on-link communication only — never routed. Routing protocols use link-local addresses as next-hops; NDP (Neighbor Discovery) relies on them entirely.
You can override the auto-generated link-local with a manual one, which is helpful when you want predictable addresses for routing adjacencies.
Router1(config-if)# ipv6 address fe80::1 link-local
That gives the interface a link-local of `fe80::1`. Peers on the same link can always reach the router at that address, regardless of which global prefix is in use. Useful for OSPFv3 next-hop configurations.
ULAs (`fc00::/7`, usually deployed as `fd00::/8`) are IPv6's answer to RFC 1918. They're not routed on the public internet and give you a stable addressing space independent of your ISP. If your ISP changes your GUA prefix, internal communication on ULAs is unaffected.
Router1(config-if)# ipv6 address fd12:3456:789a:1::1/64
Many organizations deploy ULAs alongside GUAs on the same interface — dual-addressing. Internal traffic can prefer ULA (stable, no ISP dependency), external traffic uses GUA. Applications and DNS determine which gets used.
Unlike RFC 1918, ULA prefixes are supposed to be randomly generated (RFC 4193) to avoid collisions during mergers. In practice many admins pick easy prefixes like `fd00:...` and accept the risk.
Three addresses visible: link-local (FE80::1), global unicast (2001:DB8:ACAD:1::1), and ULA (FD12:3456:789A:1::1). The interface also joined multicast groups FF02::1 (all-nodes) and FF02::2 (all-routers) automatically.
Three address types in hand. Next lesson: interface configuration in detail.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.