RFC 4291
IP Version 6 Addressing Architecture
RFC 4291: IP Version 6 Addressing Architecture 🦋
okay bestie, this is RFC 4291, and it's the one that explains how IPv6 addresses work 🌸 It was written by R. Hinden at Nokia and S. Deering at Cisco Systems, and it came out in February 2006 in the Network Working Group. It's a Standards Track document (so it's the real deal, not just an opinion piece 💅). It's also the replacement for RFC 3513, which it obsoletes, and it doesn't update anything else.
Here's the full title block in the original's format, because it's kind of iconic:
Network Working Group R. Hinden
Request for Comments: 4291 Nokia
Obsoletes: 3513 S. Deering
Category: Standards Track Cisco Systems
February 2006Status of This Memo 📋
This document is a specification for an Internet standards track protocol for the Internet community, and it asks for discussion and suggestions for improvements. If you want to know where this protocol stands in the standardization process, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🫶
Copyright Notice ©️
Copyright (C) The Internet Society (2006).Abstract ✨
This specification defines the addressing architecture of the IP Version 6 (IPv6) protocol. It covers the IPv6 addressing model, how IPv6 addresses are written as text, and the definitions of unicast, anycast, and multicast addresses. It also lists the addresses an IPv6 node is required to have 🧚
This document obsoletes RFC 3513, "IP Version 6 Addressing Architecture".
Table of Contents 🌈
1. Introduction ....................................................2
2. IPv6 Addressing .................................................2
2.1. Addressing Model ...........................................3
2.2. Text Representation of Addresses ...........................4
2.3. Text Representation of Address Prefixes ....................5
2.4. Address Type Identification ................................6
2.5. Unicast Addresses ..........................................6
2.5.1. Interface Identifiers ...............................7
2.5.2. The Unspecified Address .............................9
2.5.3. The Loopback Address ................................9
2.5.4. Global Unicast Addresses ............................9
2.5.5. IPv6 Addresses with Embedded IPv4 Addresses ........10
2.5.6. Link-Local IPv6 Unicast Addresses ..................11
2.5.7. Site-Local IPv6 Unicast Addresses ..................11
2.6. Anycast Addresses .........................................12
2.6.1. Required Anycast Address ...........................12
2.7. Multicast Addresses .......................................13
2.7.1. Pre-Defined Multicast Addresses ....................15
2.8. A Node's Required Addresses ...............................17
3. Security Considerations ........................................18
4. IANA Considerations ............................................18
5. Acknowledgements ...............................................18
6. References .....................................................18
6.1. Normative References ......................................18
6.2. Informative References ....................................18
Appendix A: Creating Modified EUI-64 Format Interface Identifiers .20
Appendix B: Changes from RFC 3513 .................................221. Introduction 🎀
okay so this section sets the scene. It defines the addressing architecture of the IP Version 6 protocol, and it gives the basic formats for the different kinds of IPv6 addresses: unicast, anycast, and multicast. Lots of main character energy here 💖
2. IPv6 Addressing 🌸
IPv6 addresses are 128-bit identifiers for interfaces and for sets of interfaces. ("Interface" means what it means in Section 2 of [IPV6].) There are three types of addresses, and they're the core of everything in this doc:
- Unicast: an identifier for a single interface. A packet sent to a unicast address gets delivered to the interface that address identifies.
- Anycast: an identifier for a set of interfaces, typically belonging to different nodes. A packet sent to an anycast address gets delivered to one of those interfaces, the "nearest" one according to the routing protocols' measure of distance 🦋
- Multicast: an identifier for a set of interfaces, typically belonging to different nodes. A packet sent to a multicast address gets delivered to all of the interfaces that address identifies.
There are no broadcast addresses in IPv6. Multicast addresses took over that job ✨
In this document, fields in addresses get specific names, like "subnet". When a name is paired with the word "ID" (like "subnet ID"), it refers to the contents of that field. When it's paired with the word "prefix" (like "subnet prefix"), it refers to the whole address from the left up to and including that field 🧚
In IPv6, all zeros and all ones are legal values for any field, unless something specifically excludes them. Prefixes are allowed to contain zero-valued fields, or even end with them, so don't panic if you see a bunch of zeros 🙈
2.1. Addressing Model 💅
IPv6 addresses of every type are assigned to interfaces, not to nodes. A unicast address refers to one interface. Since each interface belongs to exactly one node, any of that node's interface unicast addresses can be used as an identifier for the node. iconic 👑
All interfaces are required to have at least one Link-Local unicast address (see Section 2.8 for other required addresses). An interface can also have several IPv6 addresses of any type (unicast, anycast, or multicast) and any scope. Unicast addresses with a scope bigger than link-scope aren't needed for interfaces that never send or receive IPv6 packets to or from non-neighbors. That's kind of convenient for point-to-point interfaces 🫶 There's one exception to this model:
A unicast address, or a set of unicast addresses, may be assigned to multiple physical interfaces, as long as the implementation treats those physical interfaces as one interface when it presents them to the internet layer. This is useful for load-sharing over multiple physical interfaces.
Right now IPv6 keeps the IPv4 idea that one subnet prefix is tied to one link. Multiple subnet prefixes may be assigned to the same link 🌈
2.2. Text Representation of Addresses 🍓
There are three common ways to write IPv6 addresses as text strings:
- The preferred form is
x:x:x:x:x:x:x:x, where eachxis one to four hexadecimal digits representing one of the eight 16-bit pieces of the address. Examples:
ABCD:EF01:2345:6789:ABCD:EF01:2345:6789
2001:DB8:0:0:8:800:200C:417AYou don't have to write the leading zeros in each field, but every field needs at least one numeral (except for the case described in 2.).
- Lots of ways of allocating certain styles of IPv6 addresses mean you'll often see long strings of zero bits. To make those easier to write, there's a special shortcut that compresses the zeros.
::stands for one or more groups of 16 zero bits.::can only show up once in an address, and it can also be used to compress leading or trailing zeros. For example, the following addresses:
2001:DB8:0:0:8:800:200C:417A a unicast address
FF01:0:0:0:0:0:0:101 a multicast address
0:0:0:0:0:0:0:1 the loopback address
0:0:0:0:0:0:0:0 the unspecified addresscan be written like this:
2001:DB8::8:800:200C:417A a unicast address
FF01::101 a multicast address
::1 the loopback address
:: the unspecified address- Sometimes it's more convenient, especially in a mixed environment of IPv4 and IPv6 nodes, to use the form
x:x:x:x:x:x:d.d.d.d. Here, eachxis the hexadecimal value of one of the six high-order 16-bit pieces, and eachdis the decimal value of one of the four low-order 8-bit pieces (the standard IPv4 representation). Examples:
0:0:0:0:0:0:13.1.68.3
0:0:0:0:0:FFFF:129.144.52.38or in compressed form:
::13.1.68.3
::FFFF:129.144.52.382.3. Text Representation of Address Prefixes 🎀
Writing IPv6 address prefixes looks a lot like how IPv4 address prefixes get written in Classless Inter-Domain Routing (CIDR) notation [CIDR]. An IPv6 address prefix is written like this:
ipv6-address/prefix-lengthwhere:
ipv6-address is an IPv6 address in any of the notations listed
in Section 2.2.
prefix-length is a decimal value specifying how many of the
leftmost contiguous bits of the address comprise
the prefix.For example, these are all legal ways to write the 60-bit prefix 20010DB80000CD3 (hexadecimal):
2001:0DB8:0000:CD30:0000:0000:0000:0000/60
2001:0DB8::CD30:0:0:0:0/60
2001:0DB8:0:CD30::/60And these are NOT legal ways to write that prefix 🙅♀️
2001:0DB8:0:CD3/60 may drop leading zeros, but not trailing
zeros, within any 16-bit chunk of the address
2001:0DB8::CD30/60 address to left of "/" expands to
2001:0DB8:0000:0000:0000:0000:0000:CD30
2001:0DB8::CD3/60 address to left of "/" expands to
2001:0DB8:0000:0000:0000:0000:0000:0CD3When you write a node address and a prefix of that node address (like the node's subnet prefix), you can combine them like this:
the node address 2001:0DB8:0:CD30:123:4567:89AB:CDEF
and its subnet number 2001:0DB8:0:CD30::/60
can be abbreviated as 2001:0DB8:0:CD30:123:4567:89AB:CDEF/602.4. Address Type Identification 🔥
You can figure out the type of an IPv6 address from its high-order bits. Here's the breakdown:
Address type Binary prefix IPv6 notation Section
------------ ------------- ------------- -------
Unspecified 00...0 (128 bits) ::/128 2.5.2
Loopback 00...1 (128 bits) ::1/128 2.5.3
Multicast 11111111 FF00::/8 2.7
Link-Local unicast 1111111010 FE80::/10 2.5.6
Global Unicast (everything else)Anycast addresses come out of the unicast address spaces (of any scope), and they aren't syntactically different from unicast addresses. So the bits alone won't tell you which one you've got 👀
Global Unicast addresses have their general format described in Section 2.5.4. Some special-purpose subtypes of Global Unicast addresses contain embedded IPv4 addresses (for IPv4-IPv6 interoperation), and those are described in Section 2.5.5.
Future specifications may redefine one or more sub-ranges of the Global Unicast space for other purposes. Until that happens, implementations must treat all addresses that don't start with any of the prefixes listed above as Global Unicast addresses 💖
2.5. Unicast Addresses 🦋
okay so IPv6 unicast addresses are aggregatable with prefixes of arbitrary bit-length, which is giving the same vibe as IPv4 addresses under Classless Inter-Domain Routing 💅
There are several types of unicast addresses in IPv6, in particular, Global Unicast, site-local unicast (deprecated, see Section 2.5.7 🙈), and Link-Local unicast. There are also some special-purpose subtypes of Global Unicast, such as IPv6 addresses with embedded IPv4 addresses. Additional address types or subtypes can be defined in the future ✨
IPv6 nodes may have considerable or little knowledge of the internal structure of the IPv6 address, depending on the role the node plays (for instance, host versus router) 🎀. At a minimum, a node may consider that unicast addresses (including its own) have no internal structure:
| 128 bits |
+-----------------------------------------------------------------+
| node address |
+-----------------------------------------------------------------+A slightly sophisticated host (but still rather simple) may additionally be aware of subnet prefix(es) for the link(s) it is attached to, where different addresses may have different values for n:
| n bits | 128-n bits |
+-------------------------------+---------------------------------+
| subnet prefix | interface ID |
+-------------------------------+---------------------------------+Though a very simple router may have no knowledge of the internal structure of IPv6 unicast addresses, routers will more generally have knowledge of one or more of the hierarchical boundaries for the operation of routing protocols. The known boundaries will differ from router to router, depending on what positions the router holds in the routing hierarchy 🧚
Except for the knowledge of the subnet boundary discussed in the previous paragraphs, nodes should not make any assumptions about the structure of an IPv6 address 🫶
2.5.1. Interface Identifiers 💖
Interface identifiers in IPv6 unicast addresses are used to identify interfaces on a link. They are required to be unique within a subnet prefix. It is recommended that the same interface identifier not be assigned to different nodes on a link. They may also be unique over a broader scope. In some cases, an interface's identifier will be derived directly from that interface's link-layer address. The same interface identifier may be used on multiple interfaces on a single node, as long as they are attached to different subnets 🌸
Note that the uniqueness of interface identifiers is independent of the uniqueness of IPv6 addresses. For example, a Global Unicast address may be created with a local scope interface identifier and a Link-Local address may be created with a universal scope interface identifier. Lowkey mind-blowing, right? 🤯
For all unicast addresses, except those that start with the binary value 000, Interface IDs are required to be 64 bits long and to be constructed in Modified EUI-64 format 💅
Modified EUI-64 format-based interface identifiers may have universal scope when derived from a universal token (e.g., IEEE 802 48-bit MAC or IEEE EUI-64 identifiers [EUI64]) or may have local scope where a global token is not available (e.g., serial links, tunnel end-points) or where global tokens are undesirable (e.g., temporary tokens for privacy [PRIV]) 🦋
Modified EUI-64 format interface identifiers are formed by inverting the "u" bit (universal/local bit in IEEE EUI-64 terminology) when forming the interface identifier from IEEE EUI-64 identifiers. In the resulting Modified EUI-64 format, the "u" bit is set to one (1) to indicate universal scope, and it is set to zero (0) to indicate local scope. The first three octets in binary of an IEEE EUI-64 identifier are as follows:
0 0 0 1 1 2
|0 7 8 5 6 3|
+----+----+----+----+----+----+
|cccc|ccug|cccc|cccc|cccc|cccc|
+----+----+----+----+----+----+written in Internet standard bit-order, where "u" is the universal/local bit, "g" is the individual/group bit, and "c" is the bits of the company_id. Appendix A, "Creating Modified EUI-64 Format Interface Identifiers", provides examples on the creation of Modified EUI-64 format-based interface identifiers 🍓
The motivation for inverting the "u" bit when forming an interface identifier is to make it easy for system administrators to hand configure non-global identifiers when hardware tokens are not available. This is expected to be the case for serial links and tunnel end-points, for example. The alternative would have been for these to be of the form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler 0:0:0:1, 0:0:0:2, etc. Simple is iconic 👑
IPv6 nodes are not required to validate that interface identifiers created with modified EUI-64 tokens with the "u" bit set to universal are unique 🙈
The use of the universal/local bit in the Modified EUI-64 format identifier is to allow development of future technology that can take advantage of interface identifiers with universal scope 🌈
The details of forming interface identifiers are defined in the appropriate "IPv6 over <link>" specification, such as "IPv6 over Ethernet" [ETHER], and "IPv6 over FDDI" [FDDI] ✨
2.5.2. The Unspecified Address 🌙
The address 0:0:0:0:0:0:0:0 is called the unspecified address. It MUST never be assigned to any node. It indicates the absence of an address. One example of its use is in the Source Address field of any IPv6 packets sent by an initializing host before it has learned its own address 🍼
The unspecified address MUST NOT be used as the destination address of IPv6 packets or in IPv6 Routing headers. An IPv6 packet with a source address of unspecified MUST never be forwarded by an IPv6 router. Not me being strict about this one 💀
2.5.3. The Loopback Address 🔁
The unicast address 0:0:0:0:0:0:0:1 is called the loopback address. It MAY be used by a node to send an IPv6 packet to itself. It MUST NOT be assigned to any physical interface. It is treated as having Link-Local scope, and may be thought of as the Link-Local unicast address of a virtual interface (typically called the "loopback interface") to an imaginary link that goes nowhere 🫠
The loopback address MUST NOT be used as the source address in IPv6 packets that are sent outside of a single node. An IPv6 packet with a destination address of loopback MUST never be sent outside of a single node and MUST never be forwarded by an IPv6 router. A packet received on an interface with a destination address of loopback MUST be dropped 🚫
2.5.4. Global Unicast Addresses 🌍
The general format for IPv6 Global Unicast addresses is as follows:
| n bits | m bits | 128-n-m bits |
+------------------------+-----------+----------------------------+
| global routing prefix | subnet ID | interface ID |
+------------------------+-----------+----------------------------+where the global routing prefix is a (typically hierarchically-structured) value assigned to a site (a cluster of subnets/links), the subnet ID is an identifier of a link within the site, and the interface ID is as defined in Section 2.5.1 🎀
All Global Unicast addresses other than those that start with binary 000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as described in Section 2.5.1. Global Unicast addresses that start with binary 000 have no such constraint on the size or structure of the interface ID field 💅
Examples of Global Unicast addresses that start with binary 000 are the IPv6 address with embedded IPv4 addresses described in Section 2.5.5. An example of global addresses starting with a binary value other than 000 (and therefore having a 64-bit interface ID field) can be found in [GLOBAL] 🦋
2.5.5. IPv6 Addresses with Embedded IPv4 Addresses 🧸
Two types of IPv6 addresses are defined that carry an IPv4 address in the low-order 32 bits of the address. These are the "IPv4-Compatible IPv6 address" and the "IPv4-mapped IPv6 address" 🌷
2.5.5.1. IPv4-Compatible IPv6 Address 🕰️
The "IPv4-Compatible IPv6 address" was defined to assist in the IPv6 transition. The format of the "IPv4-Compatible IPv6 address" is as follows:
| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|0000| IPv4 address |
+--------------------------------------+----+---------------------+Note: The IPv4 address used in the "IPv4-Compatible IPv6 address" MUST be a globally-unique IPv4 unicast address.
The "IPv4-Compatible IPv6 address" is now deprecated because the current IPv6 transition mechanisms no longer use these addresses. New or updated implementations are not required to support this address type. This one is retired, bestie 🪦
2.5.5.2. IPv4-Mapped IPv6 Address 🗺️
A second type of IPv6 address that holds an embedded IPv4 address is defined. This address type is used to represent the addresses of IPv4 nodes as IPv6 addresses. The format of the "IPv4-mapped IPv6 address" is as follows:
| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|FFFF| IPv4 address |
+--------------------------------------+----+---------------------+See [RFC4038] for background on the usage of the "IPv4-mapped IPv6 address" 📚
2.5.6. Link-Local IPv6 Unicast Addresses 🔗
Link-Local addresses are for use on a single link. Link-Local addresses have the following format:
| 10 |
| bits | 54 bits | 64 bits |
+----------+-------------------------+----------------------------+
|1111111010| 0 | interface ID |
+----------+-------------------------+----------------------------+Link-Local addresses are designed to be used for addressing on a single link for purposes such as automatic address configuration, neighbor discovery, or when no routers are present 🏠
Routers MUST NOT forward any packets with Link-Local source or destination addresses to other links. Routers don't do that, periodt 🚧
2.5.7. Site-Local IPv6 Unicast Addresses 🏚️
Site-Local addresses were originally designed to be used for addressing inside of a site without the need for a global prefix. Site-local addresses are now deprecated as defined in [SLDEP] 😭
Site-Local addresses have the following format:
| 10 |
| bits | 54 bits | 64 bits |
+----------+-------------------------+----------------------------+
|1111111011| subnet ID | interface ID |
+----------+-------------------------+----------------------------+The special behavior of this prefix defined in [RFC3513] MUST NOT be supported in new implementations (i.e., new implementations MUST treat this prefix as Global Unicast). Existing implementations and deployments MAY continue to use this prefix. Old prefix, new rules, no drama 🌸
2.6. Anycast Addresses 🎯
Okay so an IPv6 anycast address is one that gets assigned to more than one interface, usually on different nodes 💖. The vibe is that a packet sent to it gets routed to the "nearest" interface that has that address, going by the routing protocols' own measure of distance 🦋. It's very much "closest one wins" energy ✨.
Anycast addresses come out of the unicast address space and use any of the defined unicast address formats, so they're syntactically indistinguishable from unicast addresses 🙈. When a unicast address gets assigned to more than one interface and turns into an anycast address, the nodes it's assigned to MUST be explicitly configured to know it's anycast 💅.
For any assigned anycast address, there's a longest prefix P that identifies the topological region where all the interfaces in that anycast set live 🌸. Inside the region identified by P, the anycast address MUST be maintained as its own separate entry in the routing system (people call this a "host route" 🧚). Outside that region, the anycast address MAY be aggregated into the routing entry for prefix P ✨.
Now the spicy part 🌶️. In the worst case, the prefix P of an anycast set can be the null prefix, which means the members have no topological locality at all 💀. Then the anycast address MUST be maintained as a separate routing entry across the entire Internet, and that's a severe scaling limit on how many "global" anycast sets can be supported 😭. So it's expected that support for global anycast sets may be unavailable or very restricted 🫶.
One expected use is identifying the set of routers that belong to an organization providing Internet service 👑. These addresses could be used as intermediate addresses in an IPv6 Routing header, so a packet gets delivered via a particular service provider or a sequence of them 🍓.
Some other possible uses are identifying the set of routers attached to a particular subnet, or the set of routers providing entry into a particular routing domain 🌈.
2.6.1. Required Anycast Address
The Subnet-Router anycast address is predefined, and it looks like this 🧁:
| n bits | 128-n bits |
+------------------------------------------------+----------------+
| subnet prefix | 00000000000000 |
+------------------------------------------------+----------------+The "subnet prefix" in an anycast address is the prefix that identifies a specific link 🔗. This anycast address is syntactically the same as a unicast address for an interface on the link, with the interface identifier set to zero.
Packets sent to the Subnet-Router anycast address will be delivered to one router on the subnet 📦. All routers are REQUIRED to support the Subnet-Router anycast addresses for the subnets they have interfaces on 🫶.
The Subnet-Router anycast address is meant for applications where a node needs to talk to any one router from that set 💜.
2.7. Multicast Addresses 📣
An IPv6 multicast address is an identifier for a group of interfaces, usually on different nodes 👯. An interface can belong to any number of multicast groups 🌸. Multicast addresses have this format:
| 8 | 4 | 4 | 112 bits |
+------ -+----+----+---------------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------------+The binary 11111111 at the start is how you know it's a multicast address 🔍.
The flgs field is a set of 4 flags, laid out like this:
+-+-+-+-+
flgs is a set of 4 flags: |0|R|P|T|
+-+-+-+-+- The high-order flag is reserved, and it MUST be initialized to 0. 🔒
- T = 0 means it's a permanently-assigned ("well-known") multicast address, assigned by the Internet Assigned Numbers Authority (IANA) 👑.
- T = 1 means it's a non-permanently-assigned ("transient" or "dynamically" assigned) multicast address 🦋.
- The P flag's definition and usage can be found in [RFC3306] 📖.
- The R flag's definition and usage can be found in [RFC3956] 📖.
The scop field is a 4-bit multicast scope value that limits the scope of the multicast group 🔭. The values go like this:
0 reserved
1 Interface-Local scope
2 Link-Local scope
3 reserved
4 Admin-Local scope
5 Site-Local scope
6 (unassigned)
7 (unassigned)
8 Organization-Local scope
9 (unassigned)
A (unassigned)
B (unassigned)
C (unassigned)
D (unassigned)
E Global scope
F reservedHere's the tea on each one ☕:
- Interface-Local scope spans only a single interface on a node, and it's only useful for loopback transmission of multicast 🪞.
- Link-Local multicast scope spans the same topological region as the corresponding unicast scope 🔗.
- Admin-Local scope is the smallest scope that MUST be administratively configured, meaning it isn't automatically derived from physical connectivity or other non-multicast-related configuration 🛠️.
- Site-Local scope is intended to span a single site 🏠.
- Organization-Local scope is intended to span multiple sites belonging to a single organization 🏢.
- Scopes labeled "(unassigned)" are available for administrators to define additional multicast regions 🎨.
The group ID identifies the multicast group, either permanent or transient, within the given scope 🆔. More definitions of the group ID field structure are provided in [RFC3306] 📖.
Here's the big one: the "meaning" of a permanently-assigned multicast address doesn't depend on the scope value 🧠. For example, if the "NTP servers group" gets a permanent multicast address with a group ID of 101 (hex), then:
FF01:0:0:0:0:0:0:101 means all NTP servers on the same interface
(i.e., the same node) as the sender.
FF02:0:0:0:0:0:0:101 means all NTP servers on the same link as the
sender.
FF05:0:0:0:0:0:0:101 means all NTP servers in the same site as the
sender.
FF0E:0:0:0:0:0:0:101 means all NTP servers in the Internet.Non-permanently-assigned multicast addresses only mean something inside a given scope 🫧. For example, a group identified by the non-permanent, site-local multicast address FF15:0:0:0:0:0:0:101 at one site has no relationship to a group using the same address at a different site. It's also unrelated to a non-permanent group using the same group ID with a different scope, and to a permanent group with the same group ID 🙃.
Multicast addresses MUST NOT be used as source addresses in IPv6 packets, and they MUST NOT appear in any Routing header 🚫.
Routers MUST NOT forward any multicast packets beyond of the scope indicated by the scop field in the destination multicast address 🧱.
Nodes MUST NOT originate a packet to a multicast address whose scop field contains the reserved value 0. If one of those packets is received, it MUST be silently dropped 🤫. Nodes SHOULD NOT originate a packet to a multicast address whose scop field contains the reserved value F. If such a packet is sent or received, it MUST be treated the same as packets destined to a global (scop E) multicast address 🌍.
2.7.1. Pre-Defined Multicast Addresses
These well-known multicast addresses are pre-defined 📚. The group IDs in this section are defined for explicit scope values. Using these group IDs for any other scope values, with the T flag equal to 0, is not allowed 🙅.
Here are the Reserved Multicast Addresses 🔒:
FF00:0:0:0:0:0:0:0
FF01:0:0:0:0:0:0:0
FF02:0:0:0:0:0:0:0
FF03:0:0:0:0:0:0:0
FF04:0:0:0:0:0:0:0
FF05:0:0:0:0:0:0:0
FF06:0:0:0:0:0:0:0
FF07:0:0:0:0:0:0:0
FF08:0:0:0:0:0:0:0
FF09:0:0:0:0:0:0:0
FF0A:0:0:0:0:0:0:0
FF0B:0:0:0:0:0:0:0
FF0C:0:0:0:0:0:0:0
FF0D:0:0:0:0:0:0:0
FF0E:0:0:0:0:0:0:0
FF0F:0:0:0:0:0:0:0These are reserved, and they SHALL never be assigned to any multicast group 🚷.
The All Nodes Addresses are:
FF01:0:0:0:0:0:0:1
FF02:0:0:0:0:0:0:1These identify the group of all IPv6 nodes, within scope 1 (interface-local) or 2 (link-local) 🫂.
The All Routers Addresses are:
FF01:0:0:0:0:0:0:2
FF02:0:0:0:0:0:0:2
FF05:0:0:0:0:0:0:2These identify the group of all IPv6 routers, within scope 1 (interface-local), 2 (link-local), or 5 (site-local) 🛣️.
The Solicited-Node Address is:
FF02:0:0:0:0:1:FFXX:XXXXSolicited-Node multicast addresses are computed as a function of a node's unicast and anycast addresses 🧮. You make one by taking the low-order 24 bits of an address (unicast or anycast) and appending them to the prefix FF02:0:0:0:0:1:FF00::/104. That gives a multicast address somewhere in this range:
FF02:0:0:0:0:1:FF00:0000to
FF02:0:0:0:0:1:FFFF:FFFFFor example, the Solicited-Node multicast address for the IPv6 address 4037::01:800:200E:8C6C is FF02::1:FF0E:8C6C 🌟. IPv6 addresses that differ only in their high-order bits (say, because of multiple high-order prefixes tied to different aggregations) map to the same Solicited-Node address. That's a big win because it cuts down how many multicast addresses a node has to join 🙌.
A node is REQUIRED to compute and join (on the appropriate interface) the associated Solicited-Node multicast addresses for all unicast and anycast addresses configured on the node's interfaces, whether manually or automatically 💫.
2.8. A Node's Required Addresses 🪪
A host is REQUIRED to recognize the following addresses as identifying itself 👀:
- Its required Link-Local address for each interface.
- Any additional Unicast and Anycast addresses that have been configured for the node's interfaces (manually or automatically).
- The loopback address.
- The All-Nodes multicast addresses defined in Section 2.7.1.
- The Solicited-Node multicast address for each of its unicast and anycast addresses.
- Multicast addresses of all other groups to which the node belongs.
A router is REQUIRED to recognize all the addresses a host is REQUIRED to recognize, plus the following addresses as identifying itself 🧭:
- The Subnet-Router Anycast addresses for all interfaces for which it is configured to act as a router.
- All other Anycast addresses with which the router has been configured.
- The All-Routers multicast addresses defined in Section 2.7.1.
3. Security Considerations 🔐
Okay, the security bit is short but it matters. IPv6 addressing documents don't have any direct impact on Internet infrastructure security 🛡️. Authentication of IPv6 packets is defined in [AUTH] 🔑.
4. IANA Considerations 🎀
Okay so bestie, the "IPv4-Compatible IPv6 address" is officially deprecated by this document 💀 It's retired, no cap. IANA should keep listing the address block that holds these addresses at http://www.iana.org/assignments/ipv6-address-space as "Reserved by IETF", and not reassign it for any other purpose 🔒 Here's what that entry looks like:
0000::/8 Reserved by IETF [RFC3513] [1]IANA also added a note and a link to this address block, and it's giving very clear vibes 🫶
[5] 0000::/96 was previously defined as the "IPv4-Compatible IPv6
address" prefix. This definition has been deprecated by RFC
4291.Last bit: IANA updated the references for the IPv6 Address Architecture across its registries to match, so everything lines up 🌈
5. Acknowledgements 💖
The authors wanted to give a huge shoutout to everyone who helped out: Paul Francis, Scott Bradner, Jim Bound, Brian Carpenter, Matt Crawford, Deborah Estrin, Roger Fajman, Bob Fink, Peter Ford, Bob Gilligan, Dimitry Haskin, Tom Harsch, Christian Huitema, Tony Li, Greg Minshall, Thomas Narten, Erik Nordmark, Yakov Rekhter, Bill Simpson, Sue Thomson, Markku Savela, Larry Masinter, Jun-ichiro Itojun Hagino, Tatuya Jinmei, Suresh Krishnan, and Mahmood Ali. Main character energy from all of them 🙌
6. References 📚
6.1. Normative References
[IPV6]Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
6.2. Informative References
[AUTH]Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998.[CIDR]Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, September 1993.[ETHER]Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, December 1998.[EUI64]IEEE, "Guidelines for 64-bit Global Identifier (EUI-64) Registration Authority",http://standards.ieee.org/regauth/oui/tutorials/EUI64.html, March 1997.[FDDI]Crawford, M., "Transmission of IPv6 Packets over FDDI Networks", RFC 2467, December 1998.[GLOBAL]Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global Unicast Address Format", RFC 3587, August 2003.[PRIV]Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001.[RFC3513]Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture", RFC 3513, April 2005.[RFC3306]Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6 Multicast Addresses", RFC 3306, August 2002.[RFC3956]Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address", RFC 3956, November 2004.[RFC4038]Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E. Castro, "Application Aspects of IPv6 Transition", RFC 4038, March 2005.[SLDEP]Huitema, C. and B. Carpenter, "Deprecating Site Local Addresses", RFC 3879, September 2004.
Appendix A: Creating Modified EUI-64 Format Interface Identifiers ✨
Okay so there are a bunch of ways to make Modified EUI-64 format interface identifiers, and it really depends on what your link or node is working with 🧚 This appendix walks through some of the most common ones.
Links or Nodes with IEEE EUI-64 Identifiers
Honestly the easiest glow-up 💅 The only change needed to turn an IEEE EUI-64 identifier into an interface identifier is flipping the "u" (universal/local) bit. Here's an example of a globally unique IEEE EUI-64 identifier:
|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+----------------+Here, "c" is the bits of the assigned company_id, "0" is the universal/local bit set to universal scope, "g" is the individual/group bit, and "m" is the bits of the manufacturer-selected extension identifier. The IPv6 interface identifier looks like this:
|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|cccccc1gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+----------------+The only change is inverting the value of the universal/local bit 🔄
Links or Nodes with IEEE 802 48-bit MACs
[EUI64] defines a way to make an IEEE EUI-64 identifier from an IEEE 48-bit MAC identifier. You insert two octets, with hexadecimal values of 0xFF and 0xFE (see the Note at the end of appendix), smack in the middle of the 48-bit MAC, between the company_id and the vendor-supplied id. Here's an example of a 48-bit IEEE MAC with Global scope:
|0 1|1 3|3 4|
|0 5|6 1|2 7|
+----------------+----------------+----------------+
|cccccc0gcccccccc|ccccccccmmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+Here, "c" is the bits of the assigned company_id, "0" is the universal/local bit set to Global scope, "g" is the individual/group bit, and "m" is the bits of the manufacturer-selected extension identifier. The interface identifier ends up looking like this:
|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|cccccc1gcccccccc|cccccccc11111111|11111110mmmmmmmm|mmmmmmmmmmmmmmmm|
+----------------+----------------+----------------+----------------+When IEEE 802 48-bit MAC addresses are available on an interface or a node, an implementation may use them to create interface identifiers, since they're available and unique 🙌
Links with Other Kinds of Identifiers
Some links have link-layer interface identifiers that aren't IEEE EUI-64 or IEEE 802 48-bit MACs. LocalTalk and Arcnet are examples. To make a Modified EUI-64 format identifier here, you take the link identifier (for instance, the LocalTalk 8-bit node identifier) and zero fill it to the left. So a LocalTalk 8-bit node identifier of hexadecimal value 0x4F gives you this interface identifier:
|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|0000000000000000|0000000000000000|0000000000000000|0000000001001111|
+----------------+----------------+----------------+----------------+Notice that this sets the universal/local bit to "0", which means local scope 👀
Links without Identifiers
Some links don't have any built-in identifier at all 😭 The most common ones are serial links and configured tunnels. For these, interface identifiers that are unique within a subnet prefix must be chosen.
When there's no built-in identifier on a link, the preferred approach is to use a universal interface identifier from another interface, or one that's assigned to the node itself. If you go this route, no other interface connecting the same node to the same subnet prefix may use that same identifier.
If there's no universal interface identifier available for the link, the implementation needs to create a local-scope interface identifier. The only requirement is that it be unique within a subnet prefix. There are lots of ways to pick a subnet-prefix-unique interface identifier, including the following:
- Manual Configuration
- Node Serial Number
- Other Node-Specific Token
The subnet-prefix-unique interface identifier should be generated so that it doesn't change after a node reboots or when interfaces are added or deleted from the node.
Which algorithm you pick depends on the link and the implementation. The details on forming interface identifiers are defined in the appropriate "IPv6 over <link>" specification. It is strongly recommended that a collision detection algorithm be implemented as part of any automatic algorithm 🚨
Note: [EUI-64] actually defines 0xFF and 0xFF as the bits to be inserted to create an IEEE EUI-64 identifier from an IEEE MAC-48 identifier. The 0xFF and 0xFE values are used when starting with an IEEE EUI-48 identifier. The incorrect value was used in earlier versions of the specification due to a misunderstanding about the differences between IEEE MAC-48 and EUI-48 identifiers. This document purposely continues the use of 0xFF and 0xFE because it meets the requirements for IPv6 interface identifiers (i.e., that they must be unique on the link), IEEE EUI-48 and MAC-48 identifiers are syntactically equivalent, and that it doesn't cause any problems in practice.
Appendix B: Changes from RFC 3513 🦋
Here's the tea ☕ The following changes were made from RFC 3513, "IP Version 6 Addressing Architecture":
- The restrictions on using IPv6 anycast addresses were removed. There's now enough experience with anycast addresses, the issues aren't specific to IPv6, and the GROW working group is working in this area.
- Deprecated the Site-Local unicast prefix. Changes include the following:
- Removed Site-Local from the special list of prefixes in Section 2.4.
- Split the section titled "Local-use IPv6 Unicast Addresses" into two sections, "Link-Local IPv6 Unicast Addresses" and "Site-Local IPv6 Unicast Addresses".
- Added text to the new section describing the Site-Local deprecation.
- Changes to resolve issues raised in the IAB response to the Robert Elz appeal. Changes include the following:
- Added clarification to Section 2.5 that nodes should make no assumptions about the structure of an IPv6 address.
- Changed the text in Section 2.5.1 and Appendix A to refer to the Modified EUI-64 format interface identifiers with the "u" bit set to one (1) as universal.
- Added clarification to Section 2.5.1 that IPv6 nodes are not required to validate that interface identifiers created in Modified EUI-64 format with the "u" bit set to one are unique.
- Changed the reference indicated in Section 2.5.4 "Global Unicast Addresses" to RFC 3587.
- Removed mention of NSAP addresses in examples.
- Clarified that the "x" in the textual representation can be one to four digits.
- Deprecated the "IPv6 Compatible Address" because it isn't being used in the IPv6 transition mechanisms.
- Added the "R" and "P" flags to Section 2.7 on multicast addresses, along with pointers to the documents that define them.
- Editorial changes ✏️
Authors' Addresses 📮
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA
Phone: +1 650 625-2004
EMail: bob.hinden@nokia.com
Stephen E. Deering
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USAFull Copyright Statement ✨
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📦 where this page comes from
- ietf: rfc4291 2006-02
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms