Table of contents
Table of contents
TL;DR
Speaking strictly of IPv6 address configuration, there are two (automatic) methods: stateful configuration and stateless configuration.
Stateful Configuration
Stateful configuration usually refers to Stateful DHCPv6, i.e. configuring IPv6 addresses and other network settings via DHCPv6.
The DHCPv6 protocol hands the job of configuring IPv6 addresses and other network settings (such as DNSv6 and NTPv6 servers) to the router (or rather, the DHCPv6 server). In other words, with stateful configuration, it is the router (or the DHCPv6 server) that decides how clients get configured.
To use stateful configuration, router advertisements need to carry the M flag (meaning IPv6 addresses may be configured via DHCPv6) and/or the O flag (meaning other IPv6 network settings may be configured via DHCPv6).
With DHCPv6, the router (or the DHCPv6 server) can easily manage large networks centrally. DHCPv6 lets the router decide the range of IPv6 addresses (the address pool), or hand out the same DNSv6, NTPv6 and PXEv6 servers to every client. What’s more, since DHCPv6 is a stateful protocol, the router (or the DHCPv6 server) knows which addresses have been assigned, can set lease times for IPv6 addresses, and can bind IPv6 addresses to device MAC addresses (much like DHCPv4).
The one drawback is that, by deliberate design of Android (reportedly to stop mobile carriers from using DHCPv6 to tie a user’s device to a unique IPv6 address; others say DHCPv6 might bring NAT66 back from the dead), stateful IPv6 address configuration is still unsupported as of Android 15 (although third-party solutions exist for rooted phones). iOS, by contrast, has supported it since as early as iOS 4.2.
Stateless Configuration
Stateless configuration means SLAAC (Stateless Address Autoconfiguration).
With stateless configuration, the router uses IPv6 multicast to send Router Advertisements (RA) to all IPv6 clients on the network, carrying the A flag (if it is absent, stateless configuration is not allowed), the IPv6 prefix, and IPv6 routing information.
With this method, the client decides how to configure its own IPv6 address. After receiving the IPv6 prefix, the client fills in the suffix of the address with its own 64-bit ID, and sets the device that sent the router advertisement as its default gateway. The 64-bit ID can be derived from the MAC address, or be entirely random.
Since the router has no idea how clients configure their IPv6 addresses, an IPv6 client avoids duplicate addresses by sending a Neighbor Solicitation (NS) to ask all other clients on the network. If no client responds, the address is available; the client then sends a Neighbor Advertisement (NA) to all other clients, announcing that it is the owner of that IPv6 address.
IPv6 clients can rely on a DNSv4 server for IPv6 name resolution. That said, under stateless configuration there are two ways to configure a DNSv6 server: RDNSS (Recursive DNS Server, though support is limited) and DHCPv6 (as long as the router advertisement carries the O flag). When SLAAC is used to configure IPv6 addresses, the latter is also called Stateless DHCPv6.
Closing Words
This post isn’t of much use, so just treat it as me rambling.