Stateless Address Auto Configuration

Stateless Address Auto Configuration (SLAAC) is the IPv6 Auto Configuration technique that is done automatically on the hosts with the help of IP Prefixes coming from a router with Router Advertisements (RAs). Here, EUI-64 format can also used together with the sent IP Prefix by the router. In this type of Auto configuration, in Stateless Auto Address Configuration there is no need to track any address information like Stateful, DHCP technique.

 

Learning IP Addresses from the DHCP is a Stateful Auto Address Configuration. This can be done with both IPv4 and IPv6. But with IPv6, a new term has come. This is Stateless Auto Address Configuration (SLAAC). So, this Stateless Auto Address Configuration can be done with DHCP or without DHCP.

 

In normal DHCP, DHCP Servers track the addresses that they have given. DHCP Servers has lists about these IP addresses. This is a Stateful Addresss Auto Configuration. Here, in SLAAC, there is another technique. This technique is Stateful. The addresses given to the hosts are not tracked in SLAAC.

 

DHCP itself is a Stateful technique. But when it is used with SLAAC, only some properties of DHCP is used. So, SLAAC can be used with two types according to DHCP usage. These SLAAC types are:

 

• SLAAC + DHCP Stateless Auto Configuration
• SLAAC (No DHCP) Stateless Auto Configuration

 

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Stateless Auto Address Configuration is done with the help of Router Advertisement Messages coming from the router. These Router Advertisement Messages contain some specifal flags. These are M and O flags. According to these flags, the type of Auto Configuration is determined, Stateful or Stateless auto configuration is done.

 

Here, M Flag is always 0 for SLAAC. And for SLAAC with DHCPv6, O Flag is set to 1. For SLAAC without DHCP, 0 Flag is set to 0.

SLAAC is generally used alone but SLAAC with DHCP is also used for getting another information. With SLAAC with DHCP, IPv6 addresses are advertised by the router. But here, DHCP provides other information, like DNS information to the hosts. For SLAAC+DHCP, M bit is 0 and O bit is 1.

SLAAC is also used without DHCP. Here,both M bit and O bit are 0.

 

With NDP (Neighbor Discovery Protocol), hosts learn Link prefixes from the Routers. Routers send RAs (Router Advertisements) with a Prefix. And hosts create a EUI-64 format address with the help of their MAC address. After that they combine the Prefix coming from the router and the created EUI-64 format address. This  is called “SLAAC (Stateless Address Auto Configuration)”.