A Proposal for Long-Term Expansion of the North American Numbering Plan (NANP) to 11 Digits
draft-robinson-nanp-expansion-02
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Paul Robinson | ||
| Last updated | 2026-06-23 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-robinson-nanp-expansion-02
Network Working Group P. Robinson
Internet-Draft 20 June 2026
Intended status: Informational
Expires: 22 December 2026
A Proposal for Long-Term Expansion of the North American Numbering Plan
(NANP) to 11 Digits
draft-robinson-nanp-expansion-02
Abstract
The North American Numbering Plan (NANP) is projected to exhaust
available telephone numbering resources within the coming decades
under current allocation and utilization trends. Existing mitigation
strategies, including area code overlays and number pooling, extend
the usable life of the NANP but introduce increasing operational
complexity and user confusion.
This document proposes a long-term, uniform expansion of NANP
telephone numbers from 10 to 11 digits through extension of the area
code or Numbering Plan Area (NPA) from 3 to 4 digits. The proposal
emphasizes backward compatibility, fixed-length numbering, and a
multi-phase transition strategy designed to minimize disruption.
This document is intended to stimulate discussion and does not
represent the position of any standards body or regulatory authority.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-robinson-nanp-expansion/.
Source for this draft and an issue tracker can be found at
https://github.com/electric-socket/11digitdialing.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Robinson Expires 22 December 2026 [Page 1]
Internet-Draft NANP Expansion June 2026
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 December 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Executive Summary . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Core Proposal . . . . . . . . . . . . . . . . . . . . 3
1.2. Why This Approach . . . . . . . . . . . . . . . . . . . . 4
1.3. Transition Strategy . . . . . . . . . . . . . . . . . . . 4
1.4. Alternatives Considered and Rejected . . . . . . . . . . 5
1.5. Key Risks . . . . . . . . . . . . . . . . . . . . . . . . 5
1.6. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 7
6. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Proposed Expansion Model . . . . . . . . . . . . . . . . . . 9
9. Human Factors . . . . . . . . . . . . . . . . . . . . . . . . 11
10. Routing Considerations . . . . . . . . . . . . . . . . . . . 11
11. Potential Misunderstandings . . . . . . . . . . . . . . . . . 11
12. Transition Strategy . . . . . . . . . . . . . . . . . . . . . 12
12.1. Phase 0: Infrastructure Readiness . . . . . . . . . . . 12
12.2. Phase 1: Dual-Format Acceptance . . . . . . . . . . . . 12
12.3. Phase 2: User Notification . . . . . . . . . . . . . . . 13
12.3.1. Segment 1 . . . . . . . . . . . . . . . . . . . . . 13
12.3.2. Segment 2 . . . . . . . . . . . . . . . . . . . . . 13
12.4. Phase 3: Mandatory Expansion . . . . . . . . . . . . . . 13
12.5. Phase 4: Full Expansion . . . . . . . . . . . . . . . . 13
13. Alternatives Considered . . . . . . . . . . . . . . . . . . . 13
13.1. Further subdivision of number pooling blocks . . . . . . 14
13.2. Expansion using only reserved NPA ranges (e.g., N9X) . . 14
Robinson Expires 22 December 2026 [Page 2]
Internet-Draft NANP Expansion June 2026
13.3. Overlay NPAs . . . . . . . . . . . . . . . . . . . . . . 14
13.4. Variable-length numbering schemes . . . . . . . . . . . 14
13.5. Expansion Of NXX To NXXX . . . . . . . . . . . . . . . . 15
13.6. Expansion Of XXXX To XXXXX . . . . . . . . . . . . . . . 15
14. Operational Considerations . . . . . . . . . . . . . . . . . 15
15. Backwards Compatibility Risks . . . . . . . . . . . . . . . . 16
16. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 17
17. Economic Considerations . . . . . . . . . . . . . . . . . . . 18
17.1. Advantages to Starting Early . . . . . . . . . . . . . . 19
17.2. Cost Considerations Of Methods Used . . . . . . . . . . 20
17.3. Billing and Rating Impact . . . . . . . . . . . . . . . 20
17.4. Stakeholder Impact . . . . . . . . . . . . . . . . . . . 20
18. Security Considerations . . . . . . . . . . . . . . . . . . . 20
19. Transition Governance . . . . . . . . . . . . . . . . . . . . 21
20. Transition Difficulties . . . . . . . . . . . . . . . . . . . 21
21. International Considerations . . . . . . . . . . . . . . . . 22
22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
23. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 22
24. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
24.1. Normative References . . . . . . . . . . . . . . . . . . 23
24.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24
1. Executive Summary
The North American Numbering Plan (NANP), which governs the 10-digit
telephone numbers used across the US, Canada, and partner nations,
faces eventual exhaustion of its numbering capacity. Current stopgap
measures — area code overlays and number pooling — extend the
system's lifespan but compound routing complexity and degrade user
experience.
1.1. The Core Proposal
This document proposes expanding NANP telephone numbers from 10 to 11
digits by enlarging the area code field from three digits to four
digits:
Current: NPA-NXX-XXXX Proposed: NPAX-NXX-XXXX
The central design choice is to preserve fixed-length, fixed-position
parsing. During the initial transition, the added fourth area-code
digit would be uniformly limited to either 0 or 1, because current
NANP NXX prefixes begin with digits 2 through 9. That lets switching
systems distinguish old 10-digit numbers from new 11-digit numbers by
examining the fourth digit, avoiding variable-length dialing, timing
ambiguity, or major changes to downstream routing logic.
Robinson Expires 22 December 2026 [Page 3]
Internet-Draft NANP Expansion June 2026
Rather than altering the subscriber number or prefix (NXX), the
proposal extends only the area code field. Since the NXX of a
subscriber telephone number cannot start with 0 or 1, that is used to
modify the area code (NPA), by appending it with either a 0 or a 1.
A number like 213-555-1234 would become 2130-555-1234 or
2131-555-1234. The fourth digit is initially restricted to 0 or 1,
which allows switching systems to distinguish new 11-digit numbers
from legacy 10-digit numbers by a simple positional check — no
timing-based or variable-length parsing required. In a later phase,
digits 2–9 would be unlocked, expanding total addressable area codes
roughly tenfold (from ~700 usable NPAs today to over 7,000 NPAXs).
1.2. Why This Approach
The proposal explicitly rejects several alternatives — expanding the
subscriber prefix or line number, variable-length numbering, and
relying solely on reserved NPA ranges — on grounds that each would
impose greater infrastructure cost, database bloat, or user
disruption than the NPA extension approach. The NXX and XXXX fields
are left entirely unchanged, which preserves existing routing tables
and local number portability (LNP) database schemas.
1.3. Transition Strategy
The proposal outlines a five-phase migration:
* Phase 0 — Silent infrastructure readiness (networks updated, no
public announcement)
* Phase 1 — Dual-format acceptance (both 10- and 11-digit numbers
routed simultaneously)
* Phase 2 — Public notification, first via announcement, then via
intercept warnings on legacy 10-digit calls (calls still complete)
* Phase 3 — Mandatory 11-digit dialing (legacy format calls rejected
with a SIT-tone intercept)
* Phase 4 — Full expansion (fourth digit opened to 0–9, unlocking
the complete numbering pool)
Robinson Expires 22 December 2026 [Page 4]
Internet-Draft NANP Expansion June 2026
1.4. Alternatives Considered and Rejected
The document evaluates and dismisses further number pooling
subdivision, overlay-only strategies, variable-length numbering,
expansion of the NXX to four digits, and expansion of the line number
to five digits. Each alternative either provides insufficient long-
term relief or introduces greater disruption to routing
infrastructure, portability databases, and switch hardware than the
NPAX approach.
1.5. Key Risks
Backward compatibility is the primary concern. Hard-coded 10-digit
fields in software, embedded devices (elevator phones, alarm systems,
card terminals), 911/E-911/PSAP infrastructure, and legacy PBX
systems will all require remediation. Security systems such as STIR/
SHAKEN call authentication must be updated to handle both formats
during transition. The draft stresses early planning to avoid a
time-pressured implementation.
1.6. Scope
The proposed format remains within the ITU-T E.164 15-digit maximum
and requires no change to the +1 country code. This document is an
informational discussion draft and does not represent the position of
any standards body or regulatory authority.
2. Introduction
The NANP currently utilizes a fixed-length 10-digit numbering format
(NPA-NXX-XXXX). Growth in telecommunications services, device
proliferation, and number portability has steadily increased demand
for numbering resources.
Mitigation strategies such as overlays and thousands-block number
pooling have delayed exhaustion but introduce increasing complexity
in routing, administration, and user experience.
This document explores a uniform expansion of NANP numbers to 11
digits as a long-term solution.
This proposal preserves fixed-position digit parsing, avoiding
variable-length interpretation and timing-based ambiguity. This
property is a primary design objective, as it minimizes required
changes to existing switching and routing infrastructure.
Robinson Expires 22 December 2026 [Page 5]
Internet-Draft NANP Expansion June 2026
3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
4. Definitions
CP: is Cellular Provider, the service provider for a caller using
cellular telephony.
IXC: Interexchange Carrier - the organization that carries a call
between the caller's service provider and the called party's
service provider where the caller's service provider does not
serve the area of the called party.
Jeopardy: Jeopardy is the condition where an NPA is in danger of
running out of available NXXs to use for new telephone
subscribers, requiring a freeze on the issuance of further
subscriber numbers in that NPA until more NXXs become available.
LEC: Local Exchange Carrier, the service provider for a caller using
a landline or VoIP service, the organization that provides dial
tone and carries a call to the called party where they are within
the service area of the LEC, or transfers the call to an IXC when
the called party is outside its service area.
N versus X: In the context of a telephone number, N is used to
indicate a digit that is restricted to values of 2 through 9,
while X indicates an unrestricted digit with values 0 through 9.
NPA: The area code, or first three digits of the 10-digit telephone
number. This document will use both 'area code' and 'NPA'
interchangeably, generally with NPA used when discussing technical
and implementation issues, and area code being used when
discussing issues faced by subscribers and the public.
NPAX: The new area code, or first four digits of the new 11-digit
telephone number. This proposal recommends expansion of the NPA
field by 1 digit and provides an expansion of the entire telephone
number to 11 digits.
NXX: The prefix, or digits four through six of the 10-digit
telephone number, or first three digits of the subscriber number.
This field is to remain unchanged, but is moved to digits five
through seven of the new 11-digit telephone number.
Robinson Expires 22 December 2026 [Page 6]
Internet-Draft NANP Expansion June 2026
SIT tone: A Special Information Tone (SIT) is a standardized, three-
beep audio signal (typically 950/1400/1800 Hz) played before a
recorded announcement to indicate a telephone call has failed.
Subscriber number: The portion of the telephone number following the
NPA or NPAX. It remains unchanged at seven digits, but is moved
from positions four through ten of the telephone number, to
positions five through eleven.
Telephone number: The entire number of the party to be called,
consisting of either
* On 10-digit systems, the three-digit NPA, three-digit NXX, and
four-digit XXXX.
* On 11-digit systems, the four-digit NPAX, three-digit NXX, and
four-digit XXXX.
VoIP: Voice Over IP, or telephone service where the call initiates
from or terminates via the Internet.
XXXX: The last four digits of the subscriber number, or line number,
digits seven through ten of the 10-digit telephone number. This
field is also to remain unchanged, but is moved to digits eight
through eleven of the new 11-digit telephone number.
5. Problem Statement
The finite capacity of the current NANP is not a hypothetical concern
but an eventual certainty. According to [NANPA-OCT2025] NANPA's
October 2025 NANP Exhaust Analysis [NANPA-2025-10], the NANP is
projected to exhaust available numbering resources between 2052 and
2060, depending on demand growth rates. The baseline projection,
using an average annual CO code demand of 5,213 codes, places
exhaustion in 2060. A sensitivity analysis using demand 20% above
baseline projects exhaustion as early as 2052. These projections
assume a pool of 680 usable NPAs; the remainder of the 800-NPA
address space is reserved or unavailable, including 80 NPAs
explicitly held in reserve for future NANP expansion — an implicit
acknowledgment by the standards community that structural expansion
will eventually be required.
This is an even shorter timeline than estimated in [NANPA-APR2025],
only six months earlier.
The current NANP faces several challenges:
* Finite NPA capacity under existing numbering rules
Robinson Expires 22 December 2026 [Page 7]
Internet-Draft NANP Expansion June 2026
* Fragmentation of numbering resources due to allocation practices
* Growing operational complexity in routing and database systems
* Long lead times required for major numbering plan changes
A long-term solution should address these challenges while preserving
fixed-length numbering. Variable-length telephone number schemes are
explicitly rejected: they introduce timing-based ambiguity,
complicate digit analysis at every layer of the switching and routing
stack, increase validation burden across interconnected systems, and
create inconsistent user experiences across regions and carriers.
Fixed-length numbering is a foundational property of the NANP and
must be preserved in any expansion.
All feasible approaches to expanding NANP numbering capacity
introduce some degree of disruption. The proposed expansion of the
NPA is considered the least disruptive option, as it preserves the
existing hierarchical structure of the numbering plan and minimizes
changes to subscriber numbering and routing semantics.
6. Design Goals
The proposed solution is guided by the following goals:
* Maintain fixed-length numbering
* Minimize changes to existing routing logic
* Preserve compatibility with existing numbering structures
7. Non-Goals
The following approaches are explicitly not considered desirable:
* Variable-length telephone numbers
* Region-specific numbering formats
* Frequent or repeated structural changes to the numbering plan
* Solutions requiring rapid or "flash cut" transitions
Robinson Expires 22 December 2026 [Page 8]
Internet-Draft NANP Expansion June 2026
8. Proposed Expansion Model
This document is a proposal to expand NANP numbers from 10 to 11
digits by extending the NPA from three digits to four digits. This
ensures that existing fixed-position digit parsing logic can be
extended with minimal modification, avoiding the need for timing-
based or variable-length interpretation.
Existing numbers:
NPA-NXX-XXXX
Expanded format:
NPAX-NXX-XXXX
During initial deployment, the fourth digit added to the NPA to form
the NPAX MUST be selected such that it does not conflict with
existing digit patterns used to identify the first digit of NXX
codes. Under current NANP rules, the first digit of an NXX is
restricted to values 2 through 9.
By selecting 0 or 1 for the additional NPA digit, the boundary
between the expanded NPAX and the following NXX remains unambiguous.
This allows existing digit analysis algorithms to distinguish between
legacy 10-digit and expanded 11-digit numbers using a simple
examination of the fourth digit.
Restricting the fourth digit initially to a single value minimizes
required changes to routing logic and reduces deployment cost.
This property allows existing fixed-position digit analysis logic to
be extended with minimal modification, avoiding the need for timing-
based disambiguation, interdigit timeout adjustments, or variable-
length parsing mechanisms.
A single value (0 or 1) SHALL be used consistently across all NPAs
during the initial deployment phase to ensure uniform behavior across
networks. Uniformity avoids user confusion, simplifies parsing, and
prevents mixed national behavior during migration.
Example:
213-555-1234 (legacy)
2130-555-1234 (expanded)
Or:
Robinson Expires 22 December 2026 [Page 9]
Internet-Draft NANP Expansion June 2026
303-555-1234 (legacy)
3031-555-1234 (expanded)
A telephone switching system processes a telephone number using the
following logic:
1. Scan fourth digit of number.
2. If 0 or 1, process as NPAX.
3. Else, process as NXX.
This proposal preserves the semantic structure of the number.
* The NPAX field preserves the existing role of the NPA field,
whether geographic, overlay, toll-free, or service-specific.
* NXX is still used as the routing block.
* XXXX is still the subscriber line number.
The widespread adoption of overlay area codes has fundamentally
altered the NANP environment. A return to strictly geographic, non-
overlapping area codes is no longer practical. The proposed approach
assumes the continued existence of overlays and does not attempt to
reverse this trend.
The designation of 988 as a nationwide service code required the
elimination of 7-digit dialing in affected areas, accelerating the
transition to uniform 10-digit dialing across the NANP. As a result,
this proposal does not impact legacy 7-digit dialing, as that
capability has already been largely eliminated.
This approach ensures that numbering expansion occurs at the highest
level of the NANP hierarchy, avoiding disruption to lower-level
components such as routing prefixes and subscriber numbers.
This method preserves fixed-field positional parsing, avoiding the
need for timing-based digit collection or variable-length
interpretation, which are known sources of complexity and error
in telephony systems.
Robinson Expires 22 December 2026 [Page 10]
Internet-Draft NANP Expansion June 2026
9. Human Factors
During and after transition, published numbers SHOULD be displayed in
hyphenated 4-3-4 form (e.g., 2130-555-1234). Users will find this
format similar to the existing 3-3-4 format, and are expected to
adapt to the new format following the patterns observed during the
transition to mandatory 10-digit dialing. Contact storage systems,
dialing interfaces, and automated dialing features are expected to
adapt with minimal modification due to the preserved fixed-length
structure.
10. Routing Considerations
Existing routing systems rely on fixed field positions within the
NANP number. The proposed expansion preserves the relative position
of the NXX and subscriber line number fields, allowing for minimal
modification to routing logic.
Systems that perform digit analysis MUST be updated to recognize the
NPAX format. This includes SS7-based switching systems (the
signaling protocols used to route calls across telephone networks),
SIP routing platforms, and number portability databases.
These updates are limited to recognition of the fourth digit as a
format discriminator and do not require changes to downstream routing
logic based on NXX or subscriber number.
11. Potential Misunderstandings
Historically, the leading domestic prefix "1" served a toll-alert or
toll-access function. It warned the caller, and the network, that
the call was being placed outside the local calling area and might
incur higher charges. This distinction mattered when local calls
were flat-rate or inexpensive while toll calls could be substantially
more expensive.
That function has diminished in modern wireless, VoIP, and bundled
calling environments, where calls are often rated independently of
whether the caller dialed a leading "1". Accordingly, this proposal
treats the leading "1" as an optional domestic dialing prefix or
legacy access convention, not as part of the NANP telephone number.
Under this proposal, the NANP subscriber number expands from 10
digits to 11 digits:
NPAX-NXX-XXXX
Robinson Expires 22 December 2026 [Page 11]
Internet-Draft NANP Expansion June 2026
Some legacy systems or carrier arrangements may continue to accept or
require a leading domestic prefix, commonly "1", before the NANP
number. In that case, the dialed digit sequence may contain 12
digits:
1-NPAX-NXX-XXXX
However, the leading "1" is not part of the NANP telephone number
itself. Many wireless and VoIP systems already accept calls with or
without this prefix, and this proposal does not require any change to
that behavior.
12. Transition Strategy
A phased transition is recommended:
12.1. Phase 0: Infrastructure Readiness
Networks and systems are updated to support 11-digit numbers without
public announcement. Switching systems MUST be updated to recognize
and correctly route NPAX-based numbers during Phase 0.
The restriction of the fourth digit to 0 or 1 is bridge logic. It is
not intended to be a permanent numbering constraint. During the
transition period, it permits legacy 10-digit NANP numbers and
expanded 11-digit NANP numbers to coexist without variable-length
ambiguity, simply by examining the fourth digit of the telephone
number. Once 11-digit dialing becomes mandatory and legacy 10-digit
dialing is retired for ordinary call completion, the bridge logic is
no longer required. At that point, the fourth digit of the NPAX may
be opened to additional values under the ordinary NANP assignment
process.
There SHOULD be some cross-network communication system such as a
website, a mailing list, a help desk, and/or other method for parties
involved in the conversion to report progress and to obtain
information helpful in diagnosing problems, issues and events that
may require special attention or otherwise require additional
resources for resolution. This SHOULD be provided or operated by a
neutral third-party.
12.2. Phase 1: Dual-Format Acceptance
Both 10-digit and 11-digit dialing are accepted by all CPs, IXCs, and
LECs. All originating and terminating networks MUST accept both
formats.
Robinson Expires 22 December 2026 [Page 12]
Internet-Draft NANP Expansion June 2026
12.3. Phase 2: User Notification
Phase 2 is implemented as two segments.
12.3.1. Segment 1
In Segment 1 of Phase 2, LECs, CPs, IXCs, and regulatory authorities
MUST publicize the implementation of the expansion of the NPA to an
NPAX, where the area code is expanded to four digits, and the
telephone number to eleven digits. An important highlight of the
announcement SHOULD emphasize that there will be no change to the
subscriber number. It SHOULD also state the date Segment 2 will
begin and the date that phase 3 will begin.
12.3.2. Segment 2
In Segment 2 of Phase 2, Intercept messages SHALL be imposed on
callers dialing a 10-digit phone number, and such message SHALL
inform callers dialing telephone numbers using the current 10-digit
format of upcoming requirements, MAY inform them of the digit they
need to append to the area code, and SHOULD state the date when
dialing the new 11-digit number will be required. The call SHALL
still complete.
12.4. Phase 3: Mandatory Expansion
11-digit dialing becomes required. Callers dialing the old format
10-digit number SHALL be presented with an intercept message
beginning with a SIT tone and an announcement that they must dial the
new 4-digit area code. The message MAY announce the additional digit
that MUST be dialed. The call SHALL NOT complete, and SHALL be
treated equivalently to dialing an invalid number.
12.5. Phase 4: Full Expansion
The fourth digit of the NPA is opened to all values 0 through 9,
increasing numbering capacity.
Phase 4 MUST NOT begin until 10-digit dialing has been retired for
ordinary call completion. At that point, all NANP numbers are parsed
as NPAX-NXX-XXXX, and the fourth digit is no longer needed to
distinguish legacy and expanded formats.
13. Alternatives Considered
The following alternatives were evaluated:
Robinson Expires 22 December 2026 [Page 13]
Internet-Draft NANP Expansion June 2026
13.1. Further subdivision of number pooling blocks
Further subdivision of pooling blocks provides only limited extension
while increasing database and administrative complexity.
13.2. Expansion using only reserved NPA ranges (e.g., N9X)
If the rest of the available NPAs are exhausted, this becomes a stop-
gap measure until this or some other planned expansion of the
telephone number to 11 digits.
13.3. Overlay NPAs
Originally each NPA covered one region exclusively. As more
telephone numbers were needed, more NXXs were added until no more
were available. The answer at that point is to split the NPA, take
about half the NXXs that were geographically adjacent to each other,
and assign them to the new NPA. The advantage was the subscriber
number did not change, so local seven-digit dialing was unaffected,
and the subscriber simply had to advise people that their area code
had changed. This practice worked when regions were large and the
remaining regions after the split are of a reasonable size. When
they are city-sized or smaller, splitting NPAs produces a point of
diminishing returns, where an NPA might only be part of a city.
The switch to overlays alleviated this problem as now the combined
number pool of both NPAs is available for the entire region. It also
ends the dilemma of an NPA in jeopardy status being split, with one
NPA having more than sufficient available NXXs and the other
remaining in jeopardy. Overlays were inevitable and solved the
problem of an NXX surplus/starvation NPA split result dilemma.
While overlays are useful, and it is very likely new NPAs will be
added as overlays to existing regions, they won't be enough to solve
the problem when it is not NXXs that are in jeopardy, but NPAs.
13.4. Variable-length numbering schemes
These approaches introduce significant implementation complexity,
increase validation burden across systems, and may negatively affect
user perception of numbering uniformity.
These approaches either provide limited long-term benefit or
introduce undesirable complexity.
Robinson Expires 22 December 2026 [Page 14]
Internet-Draft NANP Expansion June 2026
13.5. Expansion Of NXX To NXXX
The expansion of the subscriber number to 8 digits by increasing the
size of the NXX to four digits was considered, but it creates an even
larger disruption:
* The number of NXXs in an NPA rises from slightly under 800 to
nearly 8,000. It would vastly increase the size and complexity of
local number portability databases
* It would also increase the number and size of call routing tables
* Third parties would have to update software for holding much
larger number ranges
* The number of potential telephone numbers in an area code rises
from around 7,000,000 to nearly 70,000,000
* In most regions this would cause each area code in the state to be
many times greater than the entire population of the region served
by that NPA. Much of the additional capacity would be unusable or
wasted.
13.6. Expansion Of XXXX To XXXXX
The expansion of the line number to 5 digits adds even more
complexity and potentially even greater added costs than expanding
the NXX to 4 digits:
* Even larger number portability databases, as each NXX expands by a
factor of 10
* Local switches have to handle not potentially 10,000 lines for
each NXX they service, but 100,000
It is entirely possible switch hardware cannot support this large a
number pool, either rewiring replacement or upgrading expensive
switching equipment. It may require splitting NXXs onto multiple
switches and require even more routing changes to accommodate. In
heavily populated urban areas it might require acquisition of
additional expensive switches, real estate, and buildings.
14. Operational Considerations
The proposed expansion is designed to minimize impact on:
* Call routing systems
Robinson Expires 22 December 2026 [Page 15]
Internet-Draft NANP Expansion June 2026
* Number portability databases
* Inter-carrier signaling
However, significant updates would be required in:
* Customer-facing systems
* Validation logic
* Legacy equipment and embedded systems
A question that may arise is "Why not allow the full number range of
0-9 in the fourth digit of the telephone number to be activated
immediately, rather than the current proposal to initially only
permit a single digit?"
The rationale is cost and complexity. Initial deployment using a
single digit minimizes required routing changes. This proposal
acknowledges that significant costs would be involved in moving to an
11-digit telephone number. Eventually, the change must happen. The
fact remains, initially implementing only one digit generates the
least amount of cost increase, as only the routing logic of the
fourth digit of the telephone number is required to be changed.
Later, when digits 2-9 are enabled, the code required to check the
fourth digit can then be eliminated. Thus, this also reduces the
complexity involved in making this change.
15. Backwards Compatibility Risks
There will need to be lead time to account for, and attention placed
on informing owners of systems that may need to be replaced because
of incapacity to update them to use the new format telephone number,
such as systems using:
* Hard-coded 10-digit fields
* Regular expression validation failure
* Embedded devices
Emergency equipment that sends "final warnings" or notifications need
to be considered:
* Elevator telephones
* Audible Burglar alarms
Robinson Expires 22 December 2026 [Page 16]
Internet-Draft NANP Expansion June 2026
* Silent alarms
* Credit/debit card processing terminals in places where Internet
connections are unavailable
* Roadside motorist assistance telephones
Civil emergency response centers will have critical need to make
certain proper expansion and adjustment is done for:
* 911/E-911 routing
* PSAP systems
* ALI/ANI databases
Number portability and carrier identification databases must be
adjusted to compensate for this change:
* LNP databases are massive, and may require updates to database
schemas and query logic in both SQL and NoSQL implementations
* Key lookup structures are dependent on current NPA-NXX format
One advantage this proposal provides is it requires no change to NXX
format, preserving existing LNP granularity.
16. Benefits
The current NPA format allows a maximum of 800 NPAs to be implemented
(200-999). However, practical considerations reduce this in several
ways:
* X11 codes (211, 311, 911 etc.) are unavailable
* 988 as an additional N11-style number (and possibly others in the
future) will reduce the supply further
* N9X was reserved for future expansion.
Given these constraints, the maximum number of NPAs is probably more
like 700. While this is a large number, it eventually will be
exhausted, possibly within the foreseeable future.
Expansion of area codes to add an extra digit, and later full
expansion to 11-digits expands the numbering range by a full factor
of magnitude, to 8000 possible NPAXs. Assuming similar carve-outs
are used:
Robinson Expires 22 December 2026 [Page 17]
Internet-Draft NANP Expansion June 2026
* N11X is reserved to prevent confusion
* 988X is also reserved
* Other similar service codes to 988 (200, 766, 322, 433 etc) are
created, and those code ranges are restricted, e.g. 200X, 322X,
433X
* N9XX is reserved for future expansion
It still leaves a huge pool of available NPAXs. If the X11X, 988X,
and 10 additional service code code blocks are reserved, in addition
to N9XX, we have
* 72 NPAXs restricted for N11X protection
* 110 NPAXs restricted for 988X protection and 10 additional service
code blocks
* 800 NPAXs reserved for N9XX expansion
Leaves more than 7,000 potential NPAXs available for assignment.
This represents an approximate order-of-magnitude increase in
addressable numbering capacity compared to the current NANP
structure.
17. Economic Considerations
I am not ignoring that the change is going to be massive, and
expensive. This is why it is important to start thinking about the
issue well in advance of when action becomes mandatory, and to begin
planning this change long before it is necessary. An orderly
transition on a long timetable with reachable milestones and progress
goals will be easier and less costly than an "under the gun" rushed
change because the deadline is approaching and we can't wait.
The history of infrastructure transitions is littered with examples
of "we'll deal with it later" becoming "we're dealing with it in
crisis mode at enormous cost." The two closest analogies in the tech
world are instructive:
Robinson Expires 22 December 2026 [Page 18]
Internet-Draft NANP Expansion June 2026
* The Internet address space IPv4 exhaustion is the cautionary tale.
The internet community knew for decades that 32-bit addresses
would run out. IPv6 was standardized in 1998. Here we are in
2026 and the transition is still incomplete — largely because
nobody acted until addresses were actually scarce, by which point
the installed base was so enormous that migration became
enormously painful and expensive. The NANP is in a similar
position today: there's still time to plan, but that window won't
stay open forever.
* The problem caused by the near-universal use of six-digit date
fields in computer systems resulted in the "Y2K" crisis when
people realized date calculations were going to fail when the year
2000 arrived, possibly in a catastrophic fashion. While extreme
effort prevented a disaster scenario, it still came at enormous
cost, because the work had to be done under severe time
constraints. This would not have been necessary if it was started
earlier. Articles in trade magazines as far back as 1985 were
warning about the upcoming problem, which was mostly ignored.
Until it couldn't be.
The 20- to 40-year timeframe for the upcoming exhaustion is the sweet
spot for this kind of change: long enough that a careful, phased
transition is feasible, short enough that "we'll get to it" is
genuinely dangerous. The 20–40 year estimate is not a reason to
wait, but the reason to start now.
17.1. Advantages to Starting Early
With 20-35 years of runway:
* Vendors can absorb changes into normal product refresh cycles
rather than emergency patches
* Carriers can budget capital expenditure in advance rather than
emergency spending
* Regulators can run proper notice-and-comment processes
* Embedded systems — elevator phones, alarm dialers, card terminals
— can be replaced through natural attrition rather than forced
recall
* The standards process itself can work at a measured pace with time
for revision and refinement
* Training, documentation, and public communication can be
thoughtful rather than rushed
Robinson Expires 22 December 2026 [Page 19]
Internet-Draft NANP Expansion June 2026
With five years of runway, every single one of those becomes a
crisis.
17.2. Cost Considerations Of Methods Used
Incremental approaches distribute cost over time but increase long-
term complexity. A planned expansion incurs higher initial cost but
may reduce cumulative cost and operational burden.
Early planning enables gradual transition and reduces the risk of
emergency implementation.
17.3. Billing and Rating Impact
Carriers price call charges (including call origination and
termination payments) on
* NPA-NXX
* LATA boundaries
Other than increasing the length of the NPA and temporarily carrying
duplicate records (one for the old 3-digit NPA, and one for its
replacement NPAX during phases 1 through 3 (much of which can be
automated) these impacts are expected to be minimal due to
preservation of the NPA-NXX structure used in existing billing and
rating systems.
17.4. Stakeholder Impact
18. Security Considerations
Changes to numbering formats may impact fraud detection systems, call
validation mechanisms, and authentication processes. These impacts
SHOULD be evaluated during implementation planning.
Number spoofing detection systems, including but not limited to STIR/
SHAKEN authentication to take into account both the current, and
replacement numbers.
Call authentication frameworks, robocall enforcement tools, and
carrier reputation systems all embed assumptions about 10-digit
number formats.
Robinson Expires 22 December 2026 [Page 20]
Internet-Draft NANP Expansion June 2026
Call authentication assumptions MUST be reviewed and updated to
ensure compatibility with both legacy and expanded numbering formats.
This represents an approximate order-of-magnitude increase in
addressable numbering capacity compared to the current NANP
structure.
19. Transition Governance
Governance is arguably the hardest part of any NANP change. Prior
transitions (e.g., the 1995 area code relief plan, the 988 rollout)
would be instructive in preparation, management, and implementation
of any solution to this issue.
One or more coordinating authorities MUST be designated to manage key
aspects of the transition. These include:
* Whether the NPAX flag digit in position 4 of the telephone number
is 0 or 1
* Timeline for start and implementation of each phase and segment of
the upgrade plan
* Who will be responsible for enforcing compliance with all
mandates?
This could be a single organization, or a cooperative arrangement of
regulatory and operating companies (Carriers, Canada's CRTC, NANPA,
Regulatory authorities in other jurisdictions, U.S. FCC, etc.)
20. Transition Difficulties
Not all parties involved in the transition will necessarily act as
expeditiously as possible. Concerns over depreciation or
amortization of existing equipment and/or software will be a serious
concern to various organizations and responsible individuals. This
may result in compatibility problems with respect to
* "Long tail" legacy systems
* Private Branch Exchange (PBX) systems
* International interoperability lag
As a result, partial deployment conditions may persist long after
others are fully ready for the changes as a result of the
implementation of 11-digit telephone numbers.
Robinson Expires 22 December 2026 [Page 21]
Internet-Draft NANP Expansion June 2026
21. International Considerations
The proposed 11-digit format remains compatible with the E.164 [ITU]
maximum length of 15 digits. Coordination with international
carriers and regulatory bodies is required.
* This change poses no impact to global numbering compatibility
* It fits into the existing +1 country code model
No changes to the E.164 country code (+1) are required.
Coordination among over 25 international jurisdictions, state and
provincial regulators, and the ITU will need to be taken into
account.
22. IANA Considerations
This document has no IANA actions.
23. Conclusion
Expansion of the NANP to 11 digits represents a viable long-term
solution to numbering exhaustion. Early commitment to a transition
plan is essential to enable a controlled and gradual transition, and
avoid time-constrained or emergency implementation.
Expansion of the NANP to 11 digits is not an immediate necessity —
but it is an eventual certainty. The question is not whether this
transition will happen, but whether it will be managed or improvised.
With a projected exhaustion window of 2052 to 2060, the NANP
community is in a position that infrastructure planners rarely enjoy:
enough lead time to do this properly. A carefully designed, phased
transition executed over 10 to 20 years allows vendors to absorb
changes into normal product refresh cycles, carriers to plan capital
expenditure in advance, embedded systems to be replaced through
natural attrition, and the public to adapt gradually rather than
abruptly. Regulatory processes can proceed at a measured pace.
Standards can be refined. Mistakes can be caught and corrected
before they propagate.
Robinson Expires 22 December 2026 [Page 22]
Internet-Draft NANP Expansion June 2026
That window is not permanent. The history of infrastructure
transitions offers two instructive examples. Y2K was resolved
successfully — but only because the industry mobilized with
sufficient lead time, and it was still extraordinarily expensive and
stressful with years to spare. IPv4 address exhaustion is the
alternative outcome: a problem understood for decades, deferred until
it became acute, and still incompletely resolved thirty years after
IPv6 was standardized.
The NANP does not have to follow either path exactly. The lead time
available today makes a third outcome possible: a transition that is
planned deliberately, implemented gradually, and completed before
urgency drives the agenda. Early evaluation and commitment to a
structural solution — of which this proposal is one candidate — is
the prerequisite for that outcome.
Given the long lead times required for numbering plan changes, the
time to begin is now, while beginning is still a choice.
24. References
24.1. Normative References
[ITU] International Telecommunications Union, "The international
public telecommunication numbering plan Recommendation
ITU-T E.164", November 2010, <https://www.itu.int/rec/
dologin_pub.asp?lang=e&id=T-REC-E.164-201011-I!!PDF-
E&type=items>.
[NANPA-APR2025]
North American Numbering Plan Administrator, "April 2025
North American Numbering Plan (NANP) Exhaust Analysis",
April 2025, <https://www.nanpa.com/sites/default/
files/2025-04/April_2025_NANP_Exhaust_Analysis_Final.pdf>.
[NANPA-OCT2025]
North American Numbering Plan Administrator, "October 2025
North American Numbering Plan (NANP) Exhaust Analysis",
October 2025, <https://www.nanpa.com/sites/default/
files/2025-10/
October_2025_NANP_Exhaust_Analysis_Final.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
Robinson Expires 22 December 2026 [Page 23]
Internet-Draft NANP Expansion June 2026
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
24.2. Informative References
[RFC1394] Robinson, P., "Relationship of Telex Answerback Codes to
Internet Domains", RFC 1394, DOI 10.17487/RFC1394, January
1993, <https://www.rfc-editor.org/rfc/rfc1394>.
Appendix A. Changelog
draft-00:
* Initial Release
draft-01:
* correct misspelling: allowsexisting -> allows existing
* Explain drawbacks of some alternative strategies
* Adjust line lengths to improve ease if editing, to take
advantage of kramdown-rfc and xml2rfc automatic reformatting
draft-02:
* add executive summary
* clarify that some actions are temporary in nature ("bridge logic")
while the transition from 10-to 11-digits is in progress.
* give examples of inaction and why it should be avoided
Author's Address
Paul Robinson
Email: comments@11digitdialing.com
Robinson Expires 22 December 2026 [Page 24]