What is SLAAC (Stateless Address Autoconfiguration)?
Learn what SLAAC is, how Router Advertisements and Duplicate Address Detection work, and how IPv6 devices self-configure.
Expected Interview Answer
SLAAC (Stateless Address Autoconfiguration) is an IPv6 mechanism that lets a device generate its own globally routable IPv6 address automatically by combining a network prefix advertised by a router with a locally generated interface identifier, without needing a DHCP server to track address assignments.
A SLAAC-capable host listens for Router Advertisement (RA) messages, which routers periodically multicast or send in response to a Router Solicitation, and which contain the on-link IPv6 prefix (commonly a /64) along with flags indicating whether SLAAC, DHCPv6, or both should be used. The host then constructs its full 128-bit address by combining that /64 prefix with a 64-bit interface identifier, historically derived from the device’s MAC address using EUI-64, though modern operating systems generate randomized, privacy-preserving interface identifiers by default (RFC 4941) to avoid exposing a stable hardware-tied identifier across networks. Because no central server tracks which address was handed to which device, SLAAC is called “stateless” — the host performs Duplicate Address Detection (DAD) itself, sending a neighbor solicitation for its own tentative address to make sure no other host on the link already claimed it, before considering the address usable. SLAAC handles addressing but not other configuration like DNS servers by itself, which is why it is often paired with stateless DHCPv6 or RA-supplied RDNSS options for a complete setup.
- Lets devices self-configure a working IPv6 address with zero manual setup
- Removes the need for a central DHCP server to track address leases
- Scales effortlessly to very large networks since routers just advertise a prefix
- Supports privacy extensions so addresses need not be tied to a fixed MAC address
AI Mentor Explanation
SLAAC is like a ground announcing over the PA "this is stand Z" and then leaving each fan to pick their own seat number within it based on their ticket stub, rather than an usher personally assigning every seat. Before sitting down, a fan quickly checks that nobody else in that stand already claimed the same seat number, and only then treats the seat as truly theirs. No central desk tracks who sits where, which is exactly why SLAAC is called stateless — the network hands out the shared prefix, and each device works out and verifies its own address.
Step-by-Step Explanation
Step 1
Router advertisement
The host receives (or solicits) a Router Advertisement carrying the on-link /64 IPv6 prefix and configuration flags.
Step 2
Interface identifier generation
The host generates a 64-bit interface identifier, either via EUI-64 from its MAC address or a randomized privacy address (RFC 4941).
Step 3
Address assembly
The device combines the advertised /64 prefix with the interface identifier to form its full 128-bit tentative IPv6 address.
Step 4
Duplicate Address Detection
The host sends a neighbor solicitation for its own tentative address; if no conflict is found, the address becomes usable.
What Interviewer Expects
- Explains SLAAC combines an advertised prefix with a self-generated interface ID
- Knows Router Advertisements are the trigger, not a DHCP server
- Understands Duplicate Address Detection and why it is needed
- Aware of privacy extensions (RFC 4941) replacing EUI-64 by default on modern OSes
Common Mistakes
- Confusing SLAAC with DHCPv6 (SLAAC needs no central server tracking leases)
- Assuming SLAAC addresses are always derived from the MAC address (privacy extensions randomize this)
- Forgetting SLAAC does not by itself provide DNS server information
- Not knowing Duplicate Address Detection runs before an address is considered valid
Best Answer (HR Friendly)
“SLAAC is a way for a device to give itself a working internet address on an IPv6 network without needing to ask a central server for one. The router just broadcasts the network’s prefix, and each device works out the rest of its own address, double-checks nobody else picked the same one, and starts using it — which makes IPv6 networks very easy to scale since there is no lease-tracking server to manage.”
Code Example
# Show IPv6 addresses assigned to an interface, including SLAAC ones
ip -6 addr show eth0
# inet6 2001:db8:abcd::1a2b:3c4d:5e6f:7a8b/64 scope global temporary
# inet6 2001:db8:abcd::a1b2:c3d4:e5f6:7890/64 scope global mngtmpaddr
# Capture Router Advertisements carrying the prefix and flags
sudo tcpdump -n -i eth0 icmp6 and 'ip6[40] == 134'
# Trigger a Router Solicitation manually
sudo rdisc6 eth0Follow-up Questions
- How does SLAAC differ from stateful DHCPv6?
- What is Duplicate Address Detection and why does SLAAC rely on it?
- How do IPv6 privacy extensions (RFC 4941) change interface identifier generation?
- How does a host get DNS server information when only using SLAAC?
MCQ Practice
1. What message does a router send that enables SLAAC?
Router Advertisements carry the on-link IPv6 prefix and flags that trigger and configure SLAAC on the host.
2. Why is SLAAC called “stateless”?
Unlike stateful DHCP, SLAAC has no server maintaining a lease table — each device derives and verifies its own address.
3. What does a host do before treating a SLAAC-generated address as usable?
The host performs Duplicate Address Detection, sending a neighbor solicitation for its own tentative address to check for conflicts.
Flash Cards
What is SLAAC? — Stateless Address Autoconfiguration — a device generates its own IPv6 address from an advertised prefix, without a DHCP server.
What triggers SLAAC? — A Router Advertisement carrying the on-link /64 prefix and configuration flags.
What does a device do before using its address? — Duplicate Address Detection — verifying no other host already has the same address.
How do modern OSes generate the interface identifier? — Via randomized privacy addresses (RFC 4941) rather than a fixed EUI-64 MAC-derived value.