This provides an advantage over IPv4 ARP. An IPv6 host looking for the MAC address of another host will send a multicast request to the Solicited Node multicast group of the target IPv6 address. Since IPv6 interfaces can have any number of IPv6 addresses, a host may join multiple Solicited Node multicast groups. This group is based on the last 24 bits of its IPv6 address. Instead, every host must join a Solicited Node multicast group. ARP broadcasts a request, but IPv6 doesn't have broadcast. When a host needs to discover the MAC address of a neighbor in IPv4, it uses ARP. That means a host can create its own addressing, not based on its MAC address. Based on that, privacy extensions and random addressing were developed, and they are used by OSes to provide privacy/security. Using SLAAC a host can construct its own IPv6 addressing using its MAC address, but many people thought that this was dangerous, giving away too much information, and allowing a particular host to be tracked. So, you either misunderstand or were misinformed on a few things. You can learn all the details of neighbor discovery by reading RFC 4861. RFCs are specified in sufficient detail to allow conforming implementations to be written. The best source of truth is the relevant RFCs, both the original ones and any that are marked as having updated or obsoleted them. Even IPv6 has had significant revisions in the last ten years. A lack of response to a Neighbor Solicitation is one indicator that the neighbor is unreachable.īooks are all well and good, but very out of date books may not be so useful. It's possible that some other host may have (rightly or wrongly) claimed an address that a host wants to use. Duplicate address detection ( RFC 4862).There are other reasons why Neighbor Solicitations are necessary, as well. IPv6 addresses may also be cryptographically generated addresses ( RFC 3972), temporary privacy addresses ( RFC 4941), or in modern operating systems, stable privacy addresses ( RFC 7217).Īn address that looks like it has an EUI-64 interface ID might not actually correspond to the indicated MAC address due to explicit configuration by an administrator.īecause you can't just "convert the address back" to a MAC address, you must send a Neighbor Solicitation to determine the MAC address. That address might not be generated with the EUI-64 format specified in RFC 2464. Every node automatically generates a link-local address, but:
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