Tag Archives: slaac



IPv6 hosts can use stateless or stateful autoconfiguration. Stateless address autoconfiguration (SLAAC) uses IPv6 prefixes from Router Advertisement (RA) messages; stateful autoconfiguration uses DHCPv6.

  • Managed-Config-Flag tells the end-host to use DHCPv6 exclusively;
  • Other-Config-Flag tells the end-host to use SLAAC to get IPv6 address and DHCPv6 to get other parameters (DNS server address, for example).
  • Absence of both flags tells the end-host to use only SLAAC.

One might assume that setting managed-config-flag in RA messages forces IPv6 hosts to use DHCPv6. Wrong, the two flags are just a polite suggestion.


Address types

Address Purpose
FF00::/8 Multicast
FF70::/12 Embedded RP
FE80::/10 Link Local Unicast
FF02::1:FF00:0000/104 Solicited node address
2000::/3 Global Unicast
2001::/32 Teredo
2001:DB8::/32 Documentation rezerved
2002::/16 Automatic 6to4
FC00::/7 Unique Local

Solicited node address

In addition, IPv6 multicast uses a solicited-node group that each router must join for all of its unicast and anycast addresses. The format for solicited-node multicast addresses is


Solicited-node addresses are built from this prefix concatenated with the low-order 24 bits (128 – 104 = 24) of the corresponding unicast or anycast address. For example, a unicast address of

has a corresponding solicited-node multicast address of


Multicast address

General multicast address format
Bits 8 4 4 112
Field prefix flags scope group ID


Multicast addresses in IPv6 always begin with FF as the first octet in the address, or FF00::/8. The second octet specifies the lifetime and scope of the multicast group. Lifetime can be permanent (0000) or temporary (0001). Scope can be local to any of the following:

  • Node – 0001
  • Link – 0010
  • Site – 0101
  • Organization – 1000
  • Global – 1110

EUI-64 Address Format

Ethernet hosts and Cisco routers with Ethernet interfaces use their 48-bit MAC addresses as a seed for EUI-64 addressing. But because the MAC address is 48 bits long and the EUI-64 process makes up the last 64 bits of an IPv6 address, the host needs to derive the other 16 bits from another source. The IEEE EUI-64 standard places the hex value FFFE into the center of the MAC address for this purpose. Finally, EUI-64 sets the universal/local bit, which is the 7th bit in the Interface ID field of the address, to indicate global scope.

Here is an example. Given the IPv6 prefix 2001:128:1F:633 and a MAC address of 00:07:85:80:71:B8, the resulting EUI-64 address is


ND Functions in IPv6

Message Type  Information Sought or Sent Source Address Destination Address  ICMP Type, Code 
Router Advertisement (RA) Routers advertise their presence and link prefixes, MTU, and hop limits Router’s link-local address FF02::1 for periodic broadcasts; address of querying host for responses to an RS 134, 0
Router Solicitation (RS) Hosts query for the presence of routers on the link. Address assigned to querying interface, if assigned, or :: if not assigned FF02::2 133, 0
Neighbor Solicitation (NS) Hosts query for other nodes’ link-layer addresses. Used for duplicate address detection and to verify neighbor reachability. Address assigned to querying interface, if assigned, or :: if not assigned Solicited-node multicast address or the target node’s address, if known 135, 0
Neighbor Advertisement (NA) Sent in response to NS messages and periodically to provide information to neighbors. Configured or automatically assigned address of originating interface Address of node requesting the NA or FF02::1 for periodic advertisements 136, 0
Redirect Sent by routers to inform nodes of better next-hop routers. Link-local address of originating node Source address of requesting node 137, 0


  • two groups: error reporting messages and informational messages
  • RFC mandates configurable rate limiting of ICMPv6 error messages (ipv6 icmp error-interval – default 100 ms + 10 token buckets)


To use stateful autoconfiguration, a host sends a DHCP request to one of two well-known IPv6 multicast addresses on UDP port 547:


    • FF02::1:2, all DHCP relay agents and servers
    • FF05::1:3, all DHCP servers