DHCPv6

Stands for: Dynamic Host Configuration Protocol for IPv6

The IPv6 version of DHCP: a server leases addresses, delegates prefixes or simply hands out options such as DNS servers.

DHCPv6, now RFC 9915 (January 2026, obsoleting RFC 8415), runs beside or instead of SLAAC. It is how ISPs delegate a prefix to a subscriber's router and how a server keeps a lease for each address, but it never supplies the default router: only Router Advertisements do.

Clients send from UDP port 546 to the link-scoped group ff02::1:2 on port 547, where servers and relay agents listen. Stateful assignment is Solicit, Advertise, Request and Reply, or two messages with Rapid Commit; stateless DHCPv6 is a single Information-request for options only. Leases sit in identity associations, IA_NA for addresses and IA_PD for delegated prefixes, each with a T1 to renew with the same server and a T2 to rebind with any.

Two gaps catch people out. An address from DHCPv6 implies a /128, not the link's prefix, and there is no default-router option at all, so a host still needs Router Advertisements for its gateway and on-link prefixes. The RA's M and O flags only say DHCPv6 is available, and a held erratum notes that M=0 does not stop a client trying. Clients identify themselves with a DUID, which servers may only compare for equality.

Prefix delegation is the part ISPs rely on: a subscriber router given a /48 cuts it into /64s, one per link, and must not advertise them for longer than the delegation lasts. A rogue server on the link is the classic fault; DHCPv6-Shield drops server messages on switch ports that should not carry them. In Wireshark, dhcpv6.msgtype shows the exchange.

Also known as: dhcp6, dhcp for ipv6, dhcpv6-pd, prefix delegation, rfc 9915, rfc 8415

Sources

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