IPv6 for an IPv4 person: what's the difference between fe80:: link-local and a global address, and is SLAAC replacing DHCP?

Question

Coming from IPv4 where every interface just had one address I configured (or got from DHCP). In the NetForge lab I enabled IPv6 on a router interface and suddenly `show ipv6 interface` lists *two* addresses: an `fe80::` one I never typed, plus the `2001:db8::` global I set.

A few things I can't map back to IPv4:
1. What is the `fe80::` address for and why does it appear automatically?
2. When would a host use link-local vs the global address?
3. Everyone says "SLAAC" for auto-addressing — is that just DHCPv6 with a new name, or a totally different mechanism? When do I still need DHCPv6?

Trying to build the right mental model before I go further. Thanks!

ben31833 · Accepted Answer

Good instinct separating these out. Map it like this:

**Link-local (`fe80::/10`)** — every IPv6 interface auto-generates one the moment IPv6 comes up; you never configure it. It's only valid on that single link (routers never forward it off-segment). Its job is the plumbing IPv4 hid from you: Neighbor Discovery (NS/NA — the ICMPv6 replacement for ARP), Router Advertisements, and it's the **next-hop** for your routing table. Look at `show ipv6 route` — next hops are `fe80::` addresses, not globals.

**Global unicast (`2000::/3`, docs use `2001:db8::/32`)** — internet-routable, the equivalent of your public/routable IPv4. Hosts use link-local to talk to on-link neighbors and the router, and the global to reach anything off-link. So both coexist on one interface by design.

**SLAAC vs DHCPv6** — genuinely different, not a rename:
- *SLAAC* = **StateLess** Address AutoConfig. The router sends a Router Advertisement containing a `/64` prefix; the **host** builds its own address (prefix + a self-generated 64-bit interface ID, EUI-64 or a random/privacy ID). Router keeps zero per-host state.
- *DHCPv6* = **stateful**, a server hands out and tracks specific addresses (like IPv4 DHCP).

The RA carries two flags that decide which:
- `M` (Managed) = get your address from DHCPv6.
- `O` (Other) = address via SLAAC, but fetch extras (DNS) from DHCPv6 — this is *stateless DHCPv6*.

Classic reason you still want DHCPv6: plain SLAAC historically didn't hand out DNS well, and you need it when you want address tracking/logging. In the lab, enable IPv6 on two router interfaces, `debug ipv6 nd` and watch the RA go out — you'll see the prefix the host uses to SLAAC itself.

Shira Sivroni · Answer

Ben covered the theory; here's the troubleshooting angle that makes it stick.

Remember: **a host can ping and route with only a link-local address.** So "IPv6 is up" and "IPv6 has a usable global address" are two separate checks. When something's broken, split them:

1. `show ipv6 interface brief` — is there an `fe80::` at all? No link-local = IPv6 isn't really enabled on that interface.
2. Is there a global? If not, and you expected SLAAC, the host isn't hearing an RA. On the router the interface needs `ipv6 address ... /64` **and** IPv6 unicast-routing on so it actually sends RAs.
3. Ping the neighbor's `fe80::` — but you must add the exit interface (e.g. `ping fe80::1` won't work without specifying which link), because link-local isn't globally unique.

That last point trips up every IPv4 person: link-local addresses are only unique *per link*, so they always need a zone/interface scope. In the NetForge canvas, wire two routers, enable IPv6 on both ends, and watch the Neighbor Solicitation resolve on the cable — it's the exact same story as watching ARP resolve, just ICMPv6 instead.

IPv6 for an IPv4 person: what's the difference between fe80:: link-local and a global address, and is SLAAC replacing DHCP?

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