Stateless address autoconfiguration

SLAAC operates through the exchange of ICMPv6 Router Solicitation and Router Advertisement messages. A host sends a Router Solicitation to the all-routers multicast address, and routers respond with Router Advertisements containing the network prefix and other configuration parameters such as the default gateway and hop limit.¹ The host then forms its address by appending a 64-bit interface identifier to the advertised 64-bit prefix. The interface identifier is typically derived from the MAC address using the EUI-64 format, which inserts the bytes 0xFF and 0xFE in the middle and flips the universal/local bit.² To ensure uniqueness, the host performs Duplicate Address Detection (DAD) by sending a Neighbor Solicitation for the tentative address; if no reply is received, the address is considered unique and assigned to the interface.

Because the EUI-64 method embeds the MAC address, it can be used to track a device across networks, raising privacy concerns. To mitigate this, RFC 4941 defines Privacy Extensions for SLAAC, which generate temporary, randomized interface identifiers that change over time.³ These temporary addresses are used for outgoing connections, while the stable address remains for server-like services. Security-wise, SLAAC itself does not authenticate routers, making it vulnerable to rogue router advertisements that could redirect traffic. Mechanisms like Secure Neighbor Discovery (SEND, RFC 3971) and operational practices such as RA Guard are used to mitigate these threats.⁴

SLAAC is often contrasted with DHCPv6, which is stateful and provides additional configuration options such as DNS server addresses and domain names. While SLAAC can deliver the prefix and default route, it originally did not provide DNS information; this gap was later addressed by RFC 8106, which defines an option for DNS configuration in Router Advertisements.⁵ In practice, many networks use a combination: SLAAC for addressing and DHCPv6 for other parameters, a model known as stateless DHCPv6. SLAAC is widely deployed in home networks, mobile networks, and IoT devices due to its simplicity and lack of server dependency. It is also used in IPv6-only networks with translation mechanisms like NAT64 to reach IPv4 resources.

Beyond the basics, SLAAC has several subtle features. The 'A' flag in Router Advertisements controls whether the prefix is used for autoconfiguration; if not set, hosts must use DHCPv6. The 'O' flag indicates that other configuration parameters should be obtained from DHCPv6. SLAAC also supports the generation of link-local addresses, which are always configured automatically and are essential for neighbor discovery. In some operating systems, the interface identifier can be derived from a stable secret rather than the MAC address, as specified in RFC 7217, to provide stable but unpredictable addresses. Additionally, SLAAC can be used in point-to-point links and for assigning addresses to virtual machines, and it plays a role in the bootstrap of IPv6 in constrained IoT networks like 6LoWPAN.

EUI-64

A method to create a 64-bit interface identifier from a 48-bit MAC address by inserting 0xFF and 0xFE and flipping the universal/local bit.

Duplicate Address Detection (DAD)

A process used to verify that a tentative address is not already in use on the link before assigning it.

Router Advertisement (RA)

An ICMPv6 message sent by routers to advertise prefixes and configuration parameters to hosts.