RFC 4193

Unique Local IPv6 Unicast Addresses 💖

Okay bestie, this is RFC 4193 🎀 It's titled Unique Local IPv6 Unicast Addresses, it's from the Network Working Group, and it's a Standards Track doc dated October 2005 🌸 The authors are R. Hinden (Nokia) and B. Haberman (JHU-APL). The header doesn't list any obsoletes or updates, so it doesn't mention any. Let's get into it ✨

Status of This Memo ✨

So this doc is an Internet standards track protocol for the whole Internet community, and it's literally asking for your discussion and suggestions 💅 For the standardization state and status, you check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited, so share it with everyone 🦋

Copyright Notice 🌈

Copyright (C) The Internet Society (2005).

Abstract 🌸

This doc defines an IPv6 unicast address format that's globally unique and meant for local communications, usually inside of a site. These addresses are not expected to be routable on the global Internet, so they're not for the wider internet 🙈

Table of Contents 💜

1. Introduction ....................................................2
2. Acknowledgements ................................................3
3. Local IPv6 Unicast Addresses ....................................3
   3.1. Format .....................................................3
        3.1.1. Background ..........................................4
   3.2. Global ID ..................................................4
        3.2.1. Locally Assigned Global IDs .........................5
        3.2.2. Sample Code for Pseudo-Random Global ID Algorithm ...5
        3.2.3. Analysis of the Uniqueness of Global IDs ............6
   3.3. Scope Definition ...........................................6
4. Operational Guidelines ..........................................7
   4.1. Routing ....................................................7
   4.2. Renumbering and Site Merging ...............................7
   4.3. Site Border Router and Firewall Packet Filtering ...........8
   4.4. DNS Issues .................................................8
   4.5. Application and Higher Level Protocol Issues ...............9
   4.6. Use of Local IPv6 Addresses for Local Communication ........9
   4.7. Use of Local IPv6 Addresses with VPNs .....................10

5. Global Routing Considerations ..................................11
   5.1. From the Standpoint of the Internet .......................11
   5.2. From the Standpoint of a Site .............................11
6. Advantages and Disadvantages ...................................12
   6.1. Advantages ................................................12
   6.2. Disadvantages .............................................13
7. Security Considerations ........................................13
8. IANA Considerations ............................................13
9. References .....................................................13
   9.1. Normative References ......................................13
   9.2. Informative References ....................................14

1. Introduction 🎀

Okay so this doc defines an IPv6 unicast address format that's globally unique and meant for local communications [IPV6]. These are called Unique Local IPv6 Unicast Addresses, and the doc shortens that to Local IPv6 addresses. They're not expected to be routable on the global Internet, but they are routable inside a more limited area like a site. They may also be routed between a limited set of sites 🦋

Local IPv6 unicast addresses have these characteristics:

• Globally unique prefix (with high probability of uniqueness) 💖
• Well-known prefix, so it's super easy to filter at site boundaries 🧚
• Lets sites be combined or privately interconnected without any address conflicts or having to renumber interfaces that use these prefixes 🫶
• Independent of Internet Service Providers, and can be used for communications inside a site without any permanent or intermittent Internet connectivity 🌸
• If it's accidentally leaked outside a site via routing or DNS, there's no conflict with any other addresses 💅
• In practice, applications may treat these addresses like global scoped addresses ✨

This doc covers the format of Local IPv6 addresses, how to allocate them, and usage considerations including routing, site border routers, DNS, application support, VPN usage, and guidelines for using them for local communication inside a site. It's giving full package 🎀

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Those capitals are the official BCP 14 key words, so read them carefully 💀

2. Acknowledgements 🌷

Okay this part is the shoutout section 🫶 The idea behind creating Local IPv6 addresses has been proposed a number of times by a bunch of different people, and the authors aren't claiming exclusive credit. Credit goes to Brian Carpenter, Christian Huitema, Aidan Williams, Andrew White, Charlie Perkins, and many others. The authors also want to thank Brian Carpenter, Charlie Perkins, Harald Alvestrand, Keith Moore, Margaret Wasserman, Shannon Behrens, Alan Beard, Hans Kruse, Geoff Huston, Pekka Savola, Christian Huitema, Tim Chown, Steve Bellovin, Alex Zinin, Tony Hain, Bill Fenner, Sam Hartman, and Elwyn Davies for their comments and suggestions on this document 💜

3. Local IPv6 Unicast Addresses 🦋

3.1. Format 🎀

The Local IPv6 addresses are created using a pseudo-randomly allocated global ID. Here's the layout:

   | 7 bits |1|  40 bits   |  16 bits  |          64 bits           |
   +--------+-+------------+-----------+----------------------------+
   | Prefix |L| Global ID  | Subnet ID |        Interface ID        |
   +--------+-+------------+-----------+----------------------------+

Here's what each field does:

• Prefix: the FC00::/7 prefix that identifies Local IPv6 unicast addresses 💖
• L: set to 1 if the prefix is locally assigned. Setting it to 0 may be defined in the future. See Section 3.2 for additional information. 🌸
• Global ID: a 40-bit global identifier used to create a globally unique prefix. See Section 3.2 for additional information. 🧚
• Subnet ID: a 16-bit identifier of a subnet within the site 🫶
• Interface ID: a 64-bit Interface ID as defined in [ADDARCH] ✨

3.1.1. Background 🌈

Okay so there were a bunch of choices when picking the size of the prefix and Global ID field length. There's a direct tradeoff between having a Global ID field big enough for foreseeable future growth and not wasting too much of the IPv6 address space for no reason 🙈 A reasonable way to judge a specific field length is to compare it to a projected 2050 world population of 9.3 billion [POPUL] and the number of resulting /48 prefixes per person. Here's the range of prefix choices:

    Prefix  Global ID     Number of          Prefixes    % of IPv6
            Length        /48 Prefixes       per Person  Address Space

    /11       37           137,438,953,472     15         0.049%
    /10       38           274,877,906,944     30         0.098%
    /9        39           549,755,813,888     59         0.195%
    /8        40         1,099,511,627,776    118         0.391%
    /7        41         2,199,023,255,552    236         0.781%
    /6        42         4,398,046,511,104    473         1.563%

A very high utilization ratio of these allocations can be assumed, because the Global ID field doesn't need any internal structure, and there's no reason to aggregate the prefixes 💅

The authors think a /7 prefix, resulting in a 41-bit Global ID space (including the L bit), is a good choice 👑 It gives a huge number of assignments (i.e., 2.2 trillion) and only uses less than .8% of the total IPv6 address space. It's unlikely this space will run out. If more than this were ever needed, extra IPv6 address space could be allocated for this purpose 🥹

3.2. Global ID 🍓

The allocation of Global IDs is pseudo-random [RANDOM]. They MUST NOT be assigned sequentially or with well-known numbers. This keeps any relationship between allocations from existing, and it helps make it clear these prefixes aren't meant to be routed globally. Specifically, these prefixes are not designed to aggregate 🙅‍♀️

This doc defines one specific local method for allocating Global IDs, indicated by setting the L bit to 1. Another method, indicated by clearing the L bit, may be defined later. Apart from the allocation method, all Local IPv6 addresses behave and are treated identically 🫧

The local assignments are self-generated and don't need any central coordination or assignment, but they have an extremely high probability of being unique 🌟

3.2.1. Locally Assigned Global IDs 💖

Locally assigned Global IDs MUST be generated with a pseudo-random algorithm consistent with [RANDOM]. Section 3.2.2 describes a suggested algorithm. It's important that all sites generating Global IDs use a functionally similar algorithm so there's a high probability of uniqueness 🔥

Using a pseudo-random algorithm for Global IDs in the locally assigned prefix gives assurance that any network numbered with such a prefix is highly unlikely to clash with any other network that has another locally assigned prefix. This is a seriously useful property for scenarios like networks merging, overlapping VPN address space, or hosts moving between such networks 🦋

3.2.2. Sample Code for Pseudo-Random Global ID Algorithm 🧚

Here's the algorithm meant for locally assigned Global IDs. In each case, the resulting Global ID gets used in the appropriate prefix as defined in Section 3.2 ✨

1. Obtain the current time of day in 64-bit NTP format [NTP].
2. Obtain an EUI-64 identifier from the system running this algorithm. If an EUI-64 does not exist, one can be created from a 48-bit MAC address as specified in [ADDARCH]. If an EUI-64 cannot be obtained or created, a suitably unique identifier, local to the node, should be used (e.g., system serial number).
3. Concatenate the time of day with the system-specific identifier in order to create a key.
4. Compute an SHA-1 digest on the key as specified in [FIPS, SHA1]; the resulting value is 160 bits.
5. Use the least significant 40 bits as the Global ID.
6. Concatenate FC00::/7, the L bit set to 1, and the 40-bit Global ID to create a Local IPv6 address prefix.

This algorithm will result in a Global ID that is reasonably unique and can be used to create a locally assigned Local IPv6 address prefix 🌈

3.2.3. Analysis of the Uniqueness of Global IDs ✨

okay so picking a pseudo random Global ID is kinda like how RTP/RTCP picks an SSRC identifier 🎀 (see Section 8.1 of [RTP]). this analysis is adapted from that doc, so we're borrowing their homework 🫶

since Global IDs are chosen randomly (and independently), it's totally possible that separate networks end up with the same Global ID 😭. for any given network with one or more random Global IDs that has inter-connections to other such networks, having a total of N such IDs, the probability that two or more of these IDs collide can be approximated with this formula:

   P = 1 - exp(-N**2 / 2**(L+1))

where P is the probability of collision, N is the number of interconnected Global IDs, and L is the length of the Global ID.

the table below shows the probability of a collision for a range of connections using a 40-bit Global ID field 🧚

      Connections      Probability of Collision

          2                1.81*10^-12
         10                4.54*10^-11
        100                4.54*10^-09
       1000                4.54*10^-07
      10000                4.54*10^-05

based on this analysis, the uniqueness of locally generated Global IDs is adequate for sites planning a small to moderate amount of inter-site communication using locally generated Global IDs 💖

---

3.3. Scope Definition 🌸

by default, the scope of these addresses is global. that means they are not limited by ambiguity like the site-local addresses defined in [ADDARCH] 🙈. these prefixes are globally unique, so their applicability is greater than site-locals. their limitation is in the routability of the prefixes, which is limited to a site and any explicit routing agreements with other sites to propagate them (also see Section 4.1). also, unlike site-locals, a site may have more than one of these prefixes and use them at the same time 👑

4. Operational Guidelines 🎀

the guidelines in this section do not require any change to the normal routing and forwarding functionality in an IPv6 host or router. these are configuration and operational usage guidelines, so no new magic needed ✨

4.1. Routing 🦋

Local IPv6 addresses are designed to be routed inside of a site in the same manner as other types of unicast addresses. they can be carried in any IPv6 routing protocol without any change 💅

it is expected that they would share the same Subnet IDs with provider-based global unicast addresses, if they were being used concurrently [GLOBAL].

the default behavior of exterior routing protocol sessions between administrative routing regions must be to ignore receipt of and not advertise prefixes in the FC00::/7 block. a network operator may specifically configure prefixes longer than FC00::/7 for inter-site communication 🔥

if BGP is being used at the site border with an ISP, the default BGP configuration must filter out any Local IPv6 address prefixes, both incoming and outgoing. it must be set both to keep any Local IPv6 address prefixes from being advertised outside of the site, as well as to keep these prefixes from being learned from another site. the exception to this is if there are specific /48 or longer routes created for one or more Local IPv6 prefixes 🌈

for link-state IGPs, it is suggested that a site utilizing IPv6 local address prefixes be contained within one IGP domain or area. by containing an IPv6 local address prefix to a single link-state area or domain, the distribution of prefixes can be controlled 🧚

4.2. Renumbering and Site Merging 🍓

the use of Local IPv6 addresses in a site makes communication that uses these addresses independent of renumbering a site's provider-based global addresses 💖

when merging multiple sites, the addresses created with these prefixes are unlikely to need to be renumbered, because all of the addresses have a high probability of being unique. routes for each specific prefix would have to be configured to allow routing to work correctly between the formerly separate sites 🫶

4.3. Site Border Router and Firewall Packet Filtering 🛡️

while no serious harm will be done if packets with these addresses are sent outside of a site via a default route, it is recommended that routers be configured by default to keep any packets with Local IPv6 addresses from leaking outside of the site, and to keep any site prefixes from being advertised outside of their site 😭

site border routers and firewalls should be configured to not forward any packets with Local IPv6 source or destination addresses outside of the site, unless they have been explicitly configured with routing information about specific /48 or longer Local IPv6 prefixes. this will ensure that packets with Local IPv6 destination addresses will not be forwarded outside of the site via a default route. the default behavior of these devices should be to install a "reject" route for these prefixes. site border routers should respond with the appropriate ICMPv6 Destination Unreachable message to inform the source that the packet was not forwarded [ICMPV6]. this feedback is important to avoid transport protocol timeouts 💀

routers that maintain peering arrangements between Autonomous Systems throughout the Internet should obey the recommendations for site border routers, unless configured otherwise 👑

4.4. DNS Issues 🌸

at the present time, AAAA and PTR records for locally assigned Local IPv6 addresses are not recommended to be installed in the global DNS 🙈

for background on this recommendation: one of the concerns about adding AAAA and PTR records to the global DNS for locally assigned Local IPv6 addresses stems from the lack of complete assurance that the prefixes are unique. there is a small possibility that the same locally assigned IPv6 Local addresses will be used by two different organizations, both claiming to be authoritative, with different contents. in this scenario, it is likely there will be a connection attempt to the closest host with the corresponding locally assigned IPv6 Local address. this may result in connection timeouts, connection failures indicated by ICMP Destination Unreachable messages, or successful connections to the wrong host. due to this concern, adding AAAA records for these addresses to the global DNS is thought to be unwise 😭

reverse (address-to-name) queries for locally assigned IPv6 Local addresses MUST NOT be sent to name servers for the global DNS, due to the load that such queries would create for the authoritative name servers for the ip6.arpa zone. this form of query load is not specific to locally assigned Local IPv6 addresses; any current form of local addressing creates additional load of this kind, due to reverse queries leaking out of the site. however, since allowing such queries to escape from the site serves no useful purpose, there is no good reason to make the existing load problems worse 💅

the recommended way to avoid sending such queries to nameservers for the global DNS is for recursive name server implementations to act as if they were authoritative for an empty d.f.ip6.arpa zone and return RCODE 3 for any such query. implementations that choose this strategy should allow it to be overridden, but returning an RCODE 3 response for such queries should be the default. this is both because it will reduce the query load problem, and also because, if the site administrator has not set up the reverse tree corresponding to the locally assigned IPv6 Local addresses in use, returning RCODE 3 is in fact the correct answer 🧚

4.5. Application and Higher Level Protocol Issues 🦋

application and other higher level protocols can treat Local IPv6 addresses in the same manner as other types of global unicast addresses. no special handling is required. this type of address may not be reachable, but that is no different from other types of IPv6 global unicast address. applications need to be able to handle multiple addresses that may or may not be reachable at any point in time. in most cases, this complexity should be hidden in APIs 🎀

from a host's perspective, the difference between Local IPv6 and other types of global unicast addresses shows up as different reachability, and could be handled by default in that way. in some cases, it is better for nodes and applications to treat them differently from global unicast addresses. a starting point might be to give them preference over global unicast, but fall back to global unicast if a particular destination is found to be unreachable. much of this behavior can be controlled by how they are allocated to nodes and put into the DNS. however, it is useful if a host can have both types of addresses and use them appropriately 🌈

note that the address selection mechanisms of [ADDSEL], and in particular the policy override mechanism replacing default address selection, are expected to be used on a site where Local IPv6 addresses are configured 👑

4.6. Use of Local IPv6 Addresses for Local Communication 🍓

Local IPv6 addresses, like global scope unicast addresses, are only assigned to nodes if their use has been enabled (via IPv6 address autoconfiguration [ADDAUTO], DHCPv6 [DHCP6], or manually). they are not created automatically in the way that IPv6 link-local addresses are, and will not appear or be used unless they are purposely configured 🫶

in order for hosts to autoconfigure Local IPv6 addresses, routers have to be configured to advertise Local IPv6 /64 prefixes in router advertisements, or a DHCPv6 server must have been configured to assign them. in order for a node to learn the Local IPv6 address of another node, the Local IPv6 address must have been installed in a naming system (e.g., DNS, proprietary naming system, etc.). for these reasons, controlling their usage in a site is straightforward 💖

to limit the use of Local IPv6 addresses, the following guidelines apply:

• Nodes that are to only be reachable inside of a site: The local DNS should be configured to only include the Local IPv6 addresses of these nodes. Nodes with only Local IPv6 addresses must not be installed in the global DNS. 🙈
• Nodes that are to be limited to only communicate with other nodes in the site: These nodes should be set to only autoconfigure Local IPv6 addresses via [ADDAUTO] or to only receive Local IPv6 addresses via [DHCP6]. Note: For the case where both global and Local IPv6 prefixes are being advertised on a subnet, this will require a switch in the devices to only autoconfigure Local IPv6 addresses. 🔥
• Nodes that are to be reachable from inside of the site and from outside of the site: The DNS should be configured to include the global addresses of these nodes. The local DNS may be configured to also include the Local IPv6 addresses of these nodes. 🌸
• Nodes that can communicate with other nodes inside of the site and outside of the site: These nodes should autoconfigure global addresses via [ADDAUTO] or receive global address via [DHCP6]. They may also obtain Local IPv6 addresses via the same mechanisms. 🧚

4.7. Use of Local IPv6 Addresses with VPNs 💜

Local IPv6 addresses can be used for inter-site Virtual Private Networks (VPN) if appropriate routes are set up. because the addresses are unique, these VPNs will work reliably and without the need for translation. they have the additional property that they will continue to work if the individual sites are renumbered or merged 🌈

5. Global Routing Considerations 🌍

okay so Section 4.1 is the one that says local addresses can't be default-routed between sites 🙅‍♀️ and people in the IPv6 working group and the wider IETF got worried that sites might try to use local addresses as globally routed, provider-independent addresses anyway 😭 this section is basically the "please don't do this, here's why" talk 💅

5.1. From the Standpoint of the Internet

ngl this is the main reason it's a bad idea 🫠 local addresses have a /48 prefix, and that prefix just doesn't fit anywhere in the normal IPv6 unicast hierarchy 🌸 regular unicast addresses can be routed hierarchically all the way down to the physical subnet (link), and they only need flat routing on that one link. local addresses would need flat routing even across the wide area Internet 😬

so packets addressed to a local address could only get routed across the wide area if that /48 prefix were carried by the wide area routing protocol in use, like BGP 🚫 which goes directly against the operational assumption that long prefixes get aggregated into way fewer short prefixes, so the routing table stays small and converges quickly 📉

if a network uses both normal IPv6 addresses [ADDARCH] and local addresses, those two types will never aggregate with each other, since they differ from the most significant bit onwards 💔 and local addresses won't aggregate with each other either, because their bit patterns are random 🎲 bottom line: trying to use local address prefixes generically with the routing tech we have today would be a seriously big operational penalty 💀

5.2. From the Standpoint of a Site

okay so there are a bunch of design features in local addresses that make it way less likely people will use them as arbitrary global unicast addresses 🙅‍♀️ these include:

• The default rules to filter packets and routes make it very difficult to use local addresses for arbitrary use across the Internet 🧱 for a site to use them as general purpose unicast addresses, it would have to make sure the default rules weren't being applied by every other site and every intermediate ISP used for its current and future communication 🥲
• They are not mathematically guaranteed to be unique and they aren't registered in public databases 🫣 collisions are highly unlikely, but they're possible, and a collision can wreck the integrity of communications 💥 the lack of public registration also creates operational problems 🙃
• The addresses are allocated randomly 🎲 if a site had multiple prefixes it wanted to use globally, advertising them would cost a ton because they couldn't be aggregated 💸
• They have a long prefix (i.e., /48), so a single local address prefix doesn't give enough address space to be used exclusively by the largest organizations 🏢 not enough room, bestie 😭

6. Advantages and Disadvantages 🎀

6.1. Advantages

this approach has the following advantages ✨:

• It provides Local IPv6 prefixes that can be used independently of any provider-based IPv6 unicast address allocations 🦋 this is useful for sites that aren't always connected to the Internet, or sites that want a distinct prefix they can use to keep traffic localized inside the site 🏠
• Applications can treat these addresses in exactly the same way as any other type of global IPv6 unicast address 🫶
• Sites can be merged without any renumbering of the Local IPv6 addresses 🤝
• Sites can change their provider-based IPv6 unicast address without disrupting any communication that uses Local IPv6 addresses 🌈
• It has a well-known prefix, which makes filtering at the site boundary easy 🚧
• It can be used for inter-site VPNs 🔐
• If it's accidentally leaked outside of a site via routing or DNS, there's no conflict with any other addresses 🙌

6.2. Disadvantages

this approach has the following disadvantages 😤:

• It's not possible to route Local IPv6 prefixes on the global Internet with current routing technology 🌐 consequently, site border routers have to filter these addresses by default 🛑
• There is a very low probability of non-unique locally assigned Global IDs being generated by the algorithm in Section 3.2.3 🎰 this risk can be ignored for all practical purposes, but it does lead to a theoretical risk of clashing address prefixes 🥺

7. Security Considerations 🔒

okay this part is serious bestie, but it's still kinda fun 💅 local IPv6 addresses do NOT provide any inherent security to the nodes that use them 🙅‍♀️ they can be used with filters at site boundaries to keep local IPv6 traffic inside the site, but that's no more or less secure than filtering any other type of global IPv6 unicast address 🧱

local IPv6 addresses do allow for address-based security mechanisms, including IPsec, across end-to-end VPN connections 🔐✨

8. IANA Considerations 📋

the IANA has assigned the FC00::/7 prefix to "Unique Local Unicast" 🎀

9. References 📚

9.1. Normative References

[ADDARCH]  Hinden, R. and S. Deering, "Internet Protocol Version 6
           (IPv6) Addressing Architecture", RFC 3513, April 2003.

[FIPS]     "Federal Information Processing Standards Publication",
           (FIPS PUB) 180-1, Secure Hash Standard, 17 April 1995.

[GLOBAL]   Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
           Unicast Address Format", RFC 3587, August 2003.

[ICMPV6]   Conta, A. and S. Deering, "Internet Control Message
           Protocol (ICMPv6) for the Internet Protocol Version 6
           (IPv6) Specification", RFC 2463, December 1998.

[IPV6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.

[NTP]      Mills, D., "Network Time Protocol (Version 3)
           Specification, Implementation and Analysis", RFC 1305,
           March 1992.

[RANDOM]   Eastlake, D., 3rd, Schiller, J., and S. Crocker,
           "Randomness Requirements for Security", BCP 106, RFC 4086,
           June 2005.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

[SHA1]     Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
           (SHA1)", RFC 3174, September 2001.

9.2. Informative References

[ADDAUTO]  Thomson, S. and T. Narten, "IPv6 Stateless Address
           Autoconfiguration", RFC 2462, December 1998.

[ADDSEL]   Draves, R., "Default Address Selection for Internet
           Protocol version 6 (IPv6)", RFC 3484, February 2003.

[DHCP6]    Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and
           M. Carney, "Dynamic Host Configuration Protocol for IPv6
           (DHCPv6)", RFC 3315, July 2003.

[POPUL]    Population Reference Bureau, "World Population Data Sheet
           of the Population Reference Bureau 2002", August 2002.

[RTP]      Schulzrinne, H., Casner, S., Frederick, R., and V.
           Jacobson, "RTP: A Transport Protocol for Real-Time
           Applications", STD 64, RFC 3550, July 2003.

---

Authors' Addresses 💌

Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA

Phone: +1 650 625-2004
EMail: bob.hinden@nokia.com


Brian Haberman
Johns Hopkins University
Applied Physics Lab
11100 Johns Hopkins Road
Laurel, MD 20723
USA

Phone: +1 443 778 1319
EMail: brian@innovationslab.net

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