CompTIA Network+ Practice Test 2025 - Free Network+ Practice Questions and Exam Prep Guide

SLAAC, or Stateless Address Autoconfiguration, obtains the local subnet prefix through Router Advertisement (RA) messages. When a device connects to an IPv6 network, it listens for these Router Advertisement messages from routers within the network. These messages contain important information, including the network's prefix, which allows the device to configure its own IP address automatically.

Router Advertisements are part of the Neighbor Discovery Protocol in IPv6, which helps with various functions such as address resolution and router discovery. The presence of these RAs essentially informs devices of the local subnet's prefix so they can generate a unique IP address. This process is efficient for auto-configuring devices without relying on manual configuration or additional protocols like DHCP.

The other options do not apply directly to how SLAAC obtains the prefix. While manual configuration could technically provide an IPv6 address, it does not facilitate auto-configuration that SLAAC is designed for. Scanning the network for addresses does not yield the subnet prefix and would be an inefficient way to configure addresses. Lastly, while DHCP can be used for IPv6 in a stateful manner (DHCPv6), SLAAC itself operates independently of this mechanism, focusing instead on the essential prefix provided in Router Advertisements.