Editing IPv6 (section)

===Address uniqueness and router solicitation===
IPv6 uses a new mechanism for mapping IP addresses to link-layer addresses (e.g. [[MAC address]]es), because it does not support the [[Broadcasting (networking)|broadcast]] addressing method, on which the functionality of the [[Address Resolution Protocol]] (ARP) in IPv4 is based. IPv6 implements the [[Neighbor Discovery Protocol]] (NDP, ND) in the [[link layer]], which relies on [[ICMPv6]] and [[multicast]] transmission.<ref name="Rosen kernel networking"/>{{rp|210}} IPv6 hosts verify the uniqueness of their IPv6 addresses in a [[local area network]] (LAN) by sending a neighbor solicitation message asking for the link-layer address of the IP address. If any other host in the LAN is using that address, it responds.<ref name="T. Narten pp. 54">{{cite journal|first=T.|last=Narten|title=Neighbor discovery and stateless autoconfiguration in IPv6|journal=IEEE Internet Computing|volume=3|issue=4|pages=54–62|date=August 1999|doi=10.1109/4236.780961}}</ref>

A host bringing up a new IPv6 interface first generates a unique link-local address using one of several mechanisms designed to generate a unique address. Should a non-unique address be detected, the host can try again with a newly generated address. Once a unique link-local address is established, the IPv6 host determines whether the LAN is connected on this link to any [[Router (computing)|router]] interface that supports IPv6. It does so by sending out an ICMPv6 router solicitation message to the all-routers<ref name="rfc4861sec637">{{Cite web |last=Narten |first=T. |date=September 2007 |title=Neighbor Discovery for IP version 6 (IPv6) |url=https://datatracker.ietf.org/doc/html/rfc4861#section-6.3.7 |url-status=live |archive-url=https://web.archive.org/web/20240117035643/https://datatracker.ietf.org/doc/html/rfc4861#section-6.3.7 |archive-date=17 January 2024 |publisher=[[IETF]] |at=section 6.3.7 |doi=10.17487/RFC4861 |rfc=4861 |doi-access=free }}</ref> multicast group with its link-local address as source. If there is no answer after a predetermined number of attempts, the host concludes that no routers are connected. If it does get a response, known as a router advertisement, from a router, the response includes the network configuration information to allow establishment of a globally unique address with an appropriate unicast network prefix.<ref name="rfc4862sec551">{{Cite web |last=Thomson |first=S. |date=September 2007 |title=IPv6 Stateless Address Autoconfiguration - Section 5.5.1 |url=https://datatracker.ietf.org/doc/html/rfc4862#section-5.5.1 |url-status=live |archive-url=https://web.archive.org/web/20240111084216/https://datatracker.ietf.org/doc/html/rfc4862#section-5.5.1 |archive-date=11 January 2024 |publisher=[[IETF]] |doi=10.17487/RFC4862 |rfc=4862 }}</ref> There are also two flag bits that tell the host whether it should use DHCP to get further information and addresses:
*The Manage bit, which indicates whether or not the host should use DHCP to obtain additional addresses rather than rely on an auto-configured address from the router advertisement.
*The Other bit, which indicates whether or not the host should obtain other information through DHCP. The other information consists of one or more prefix information options for the subnets that the host is attached to, a lifetime for the prefix, and two flags:<ref name="T. Narten pp. 54"/>
**On-link: If this flag is set, the host will treat all addresses on the specific subnet as being on-link and send packets directly to them instead of sending them to a router for the duration of the given lifetime.
**Address: This flag tells the host to actually create a global address.