Stateless Address Autoconfiguration and DHCPv6

IPv6 can assign addresses without a DHCP server. A host boots, sends a Router Solicitation, gets a Router Advertisement with a prefix, and generates its own interface ID. No server, no lease, no `ipconfig /release`. That's SLAAC, and it's why IPv6 works out of the box on most consumer networks.

#How SLAAC Builds an Address#link

1. Host sends Router Solicitation to ff02::2 (all-routers multicast)
2. Router replies with RA containing its prefix (e.g. 2001:db8:acad:1::/64)
3. Host takes the first 64 bits from the RA and generates the last 64 bits
4. The interface ID is derived from EUI-64 or privacy extensions
5. Host runs Duplicate Address Detection, then the address is usable

EUI-64 takes the host's MAC address (48 bits), inserts `ff:fe` in the middle, and flips the 7th bit (Universal/Local) to get a 64-bit interface ID. It's deterministic — the same MAC always produces the same interface ID, which means hosts are trackable across networks.

Privacy Extensions (RFC 4941) fix that. Instead of EUI-64, the host generates a random interface ID that rotates periodically. The temporary addresses are what's used for outbound connections; the EUI-64 derived address (if present) is used for inbound reachability. Most modern OSes enable privacy extensions by default.

#SLAAC vs. DHCPv6#link

#Configuring Each Mode on Cisco IOS#link

The `no-autoconfig` on the prefix in stateful mode tells hosts not to SLAAC-generate from that prefix. They must ask DHCPv6 instead. That's how you get stateful behavior — the managed-config-flag signals it, and no-autoconfig enforces it.

#Verification#link

Each client gets a DUID (DHCP Unique Identifier) — like a MAC but cryptographically constructed and stable across reboots. The binding shows the assigned address and lifetime information. Compare against `show ipv6 dhcp pool` for pool status.

#Which Mode Should You Choose?#link

#Quick Check#link