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Question

What are the three methods for dynamically assigning an IPv6 GUA (Global Unicast Address) to a host, and how do they differ?

Answer

SLAAC (Stateless Address Autoconfiguration) only (stateless, no server), SLAAC with stateless DHCPv6 (Dynamic Host Configuration Protocol version 6) (address from SLAAC, extra info from server), and stateful DHCPv6 (everything from server).

One RA branching into SLAAC-only, SLAAC+stateless DHCPv6, and stateful DHCPv6 with their flags.

* Three IPv6 GUA assignment methods by RA (Router Advertisement) flags. *

The parts of an IPv6 global unicast address: global routing prefix, subnet ID and interface ID.

* IPv6 global unicast address structure. — Michel Bakni, CC BY-SA 4.0, via Wikimedia Commons. *

The three GUA assignment methods compared:

Method	Address Source	Other Info Source	Server Tracks Addresses?
SLAAC Only	Host creates its own GUA from RA prefix	RA message (DNS (Domain Name System), MTU (Maximum Transmission Unit), gateway)	No
SLAAC + Stateless DHCPv6	Host creates its own GUA from RA prefix	DHCPv6 server (DNS, domain name)	No
Stateful DHCPv6	DHCPv6 server assigns the GUA	DHCPv6 server (everything)	Yes

Key concepts:

All three methods start with ICMPv6 (Internet Control Message Protocol version 6) Router Advertisement (RA) messages — the RA tells the host which method to use
"Stateless" means no server is keeping track of which IPv6 addresses are assigned to which clients
"Stateful" means the DHCPv6 server maintains a binding table of address-to-client mappings (like DHCPv4 (Dynamic Host Configuration Protocol version 4))

Tip: Think of it as a spectrum: SLAAC only = fully autonomous, stateless DHCPv6 = autonomous addressing + server extras, stateful DHCPv6 = server controls everything.

Go deeper:

IPv6 address (Wikipedia) — covers SLAAC, stateful/stateless DHCPv6 and how GUAs are formed from prefix + interface ID.

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Question

What are the three flags in an ICMPv6 (Internet Control Message Protocol version 6) RA (Router Advertisement) message, and what does each one tell the host to do?

Answer

A flag = use SLAAC (Stateless Address Autoconfiguration) for addressing, O flag = get other config from DHCPv6 (Dynamic Host Configuration Protocol version 6), M flag = get managed (full) config from DHCPv6.

Decision tree: M=1 stateful; else O=1 SLAAC+stateless; else SLAAC only.

* RA flag decision tree (M then O). *

The ICMPv6 Router Advertisement message format, including the M and O flag bits.

* ICMPv6 Router Advertisement message format. — Michel Bakni, CC BY-SA 4.0, via Wikimedia Commons. *

Flag	Name	When Set to 1
A	Address Autoconfiguration	Host should use SLAAC to create its own IPv6 GUA (Global Unicast Address)
O	Other Configuration	Host should contact a stateless DHCPv6 server for additional info (DNS (Domain Name System), domain name)
M	Managed Address Configuration	Host should contact a stateful DHCPv6 server for all IPv6 config

Flag combinations and their meaning:

A	O	M	Method
1	0	0	SLAAC only (default)
1	1	0	SLAAC + Stateless DHCPv6
0	0	1	Stateful DHCPv6 only

Tip: A = Address yourself, O = get Other stuff from a server, M = let the server Manage everything. The default is A=1, O=0, M=0 — pure SLAAC.

Go deeper:

RFC 4861 — Neighbor Discovery for IPv6 — defines the M, O (RA header) and A (prefix option) flags normatively.

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