In addition, IPv6 stack implementations are expected to support address autoconfiguration such as SLAAC and/or stateful DHCPv6. First, from a standards perspective, any fewer than 64 bits does not constitute a proper IID as defined in RFC 4291. If that 64-bit value sounds familiar, it should: A /64 prefix is also the smallest prefix (i.e., the least number of host bits) we are advised to configure on an interface. But what defines an IID itself? The basic definition is quite simple: In IPv6, the rightmost 64 bits of the most commonly encountered address types (e.g., global unicast (2000::/3), link-local (fe80::/10)) is reserved as an interface identifier. Thus, by comparison, the definition of IPv6 IIDs (in RFC 4291, IP Version 6 Addressing Architecture) explicitly states that they exist to “identify interfaces on a link.” What’s conspicuously missing from this definition is any characterization of those interfaces as necessarily belonging to multiple endpoints or nodes.Īs mentioned, IPv6 interface identifiers are used to identify interfaces on a link. While this wasn’t yet a common operational configuration when IPv6 was being created, beginning in the early 1990s, the designers of IPv6 recognized the future architectural benefit and flexibility of decoupling the host portion of a unicast address from the more restrictive concept of a single network host with only one network interface. A contemporary example of this would be the multiple network interfaces found on a typical mobile phone e.g., for both Wi-Fi and for 4G cellular data service. Over time of course it became more common for an endpoint to have and use more than one network interface (along with more than one unicast IPv4 address). This one-interface-per-host could be logically understood as a single unique network entity not requiring more than one unique address (with one unique host portion within that address). The difference in nomenclature could perhaps be understood as a historical artifact of the fact that a network endpoint had only one network interface. The host portion of the address is defined by the quantity and value of the rightmost bits that are left over after defining the quantity and value of the leftmost bits (i.e., the network portion of the address). In IPv4, it’s common to refer to the network and host portions of a unicast address. Therefore, the part of the IPv4 address identifying the endpoint had a one-to-one relationship with that endpoint. So why do we have an identifier in IPv6 explicitly but not in IPv4? A simplified explanation is that unicast IPv4 addresses were defined and deployed at a time when it was uncommon for a network endpoint (referred to as a node in IPv6) to have more than one network interface. In this IPv6 back-to-basics blog, we’ll take a closer look at IPv6 interface identifiers along with how they are configured and used in operations.Ī Brief History of IPv6 IIDs (or How I Learned to Stop Worrying and Decouple the Identification of Nodes from the Identification of Interfaces) Among other benefits, this facilitates the use of multicast instead of broadcast and thus reduces the volume of unnecessary traffic allowing switches and routers to operate more efficiently.Īnother example of a characteristic unique to IPv6 (and offering an operational benefit) is the concept and definition of an interface identifier (or IID) as part of the overall IPv6 address. But many elements did and the best of these exist to make deployment and management of the protocol easier and more efficient.įor example, IPv6 Neighbor Discovery provides automated layer 3 address management and communication on any single network segment. The original designers of IPv6 made sure to take advantage of the opportunity to rebuild the Internet Protocol from the “ground up.” Of course, not everything they proposed has yet found its way into typical network operations. Besides the virtually unlimited address space (a quantity that experts have argued will likely outlast the lifespan of the protocol itself), IPv6 offers many unique characteristics and features that are not found in IPv4 (i.e., the legacy Internet Protocol).
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