IPv4 address exhaustion
IPv4 address exhaustion refers to the exhaustion of unallocated Internet Protocol Version 4 (IPv4) addresses which are free and can be assigned for use on the internet. The exhaustion of IPv4 is the reason for the development and deployment of its successor, IPv6. IPv4 exhaustion has been anticipated since the late 1980s.
IP address allocation is managed by the Internet Assigned Numbers Authority (IANA) globally, and by five regional Internet registries (RIR) responsible in their designated territories for assignment to end users and local Internet registries, such as Internet service providers. On 31 January 2011, IANA announced it had exhausted its free pool of IPv4 addresses (from which IP blocks were allocated to regional RIRs), the exhaustion of the RIRs APNIC on 15 April 2011 and RIPE NCC on 14 September 2012, some parts of the world have already exhausted their IPv4 allocations, and the remaining RIRs are expected to deplete their pools within a few years.
IPv4 provides approximately 4.294 billion addresses; a subset of these have been distributed by IANA to the RIRs in blocks of approximately 16.8 million addresses each. The depletion of the IPv4 allocation pool has been a concern since the late 1980s, when the Internet started to experience dramatic growth. The Internet Engineering Task Force (IETF) created the Routing and Addressing Group (ROAD) in November 1991 to respond to the scalability problem caused by the classful network allocation system in place at the time. The anticipated shortage has been the driving factor in creating and adopting several new technologies, including Classless Inter-Domain Routing (CIDR) in 1993, network address translation (NAT), and IPv6 in 1998; IPv6 (Internet Protocol Version 6), which can support about 3.4×1038 addresses, is the IETF's successor technology to IPv4.
IP addressing 
Every node of an Internet Protocol (IP) network, such as a computer, router, or network printer, is assigned an IP address that is used to locate and identify the node in communications with other nodes on the network. Internet Protocol version 4 provides 232 (4,294,967,296) addresses. However, large blocks of IPv4 addresses are reserved for special uses and are unavailable for public allocation.
The IPv4 addressing structure provides an insufficient number of publicly routable addresses to provide a distinct address to every Internet device or service. This problem has been mitigated for some time by changes in the address allocation and routing infrastructure of the Internet. The transition from classful network addressing to Classless Inter-Domain Routing delayed the exhaustion of addresses substantially.
In addition, network address translation (NAT) permits Internet service providers and enterprises to masquerade private network address space with only one publicly routable IPv4 address on the Internet interface of a customer premise router, instead of allocating a public address to each network device. Complicating matters, IPv6-unaware NAT devices break native and 6to4 IPv6 connectivity, and a large fraction break 6in4 tunnels.
Address depletion 
While the primary reason for IPv4 address exhaustion is insufficient design capacity of the original Internet infrastructure, several additional driving factors have aggravated the shortcomings. Each of them increased the demand on the limited supply of addresses, often in ways unanticipated by the original designers of the network.
- Mobile devices
- As IPv4 increasingly became the de facto standard for networked digital communication and the cost of embedding substantial computing power into hand-held devices dropped, mobile phones have become viable Internet hosts. New specifications of 4G devices require IPv6 addressing.
- Always-on connections
- Throughout the 1990s, the predominant mode of consumer Internet access was telephone modem dial-up. The rapid growth of the dial-up networks increased address consumption rates, although it was common that the modem pools, and as a result, the pool of assigned IP addresses, were shared amongst a larger customer base. By 2007, however, broadband Internet access had begun to exceed 50% penetration in many markets. Broadband connections are always active, as the gateway devices (routers, broadband modems) are rarely turned off, so that the address uptake by Internet service providers continued at an accelerating pace.
- Internet demographics
- There are hundreds of millions of households in the developed world. In 1990, only a small fraction of these had Internet connectivity. Just 15 years later, almost half of them had persistent broadband connections. The many new Internet users in countries such as China and India are also driving address exhaustion.
- Inefficient address use
- Organizations that obtained IP addresses in the 1980s were often allocated far more addresses than they actually required, because the technology had been developed in the context of military testing and networking, and was not intended to support the scale of use seen outside that context. The initial classful network allocation method was therefore inadequate to reflect "real world" usage for a global network. For example, large companies or universities were assigned class A address blocks with over 16 million IPv4 addresses each, because the next smaller allocation unit, a class B block with 65536 addresses, was too small for their intended deployments, and because NAT (routing many private addresses via one or few public addresses) had not been developed at that time.
- Many organizations continue to utilize public IP addresses for devices not accessible outside their local network. From a global address allocation viewpoint, this is inefficient in many cases, but scenarios exist where this is preferred in the organizational network implementation strategies.
- Due to inefficiencies caused by subnetting, it is difficult to use all addresses in a block. The host-density ratio, as defined in RFC 3194, is a metric for utilization of IP address blocks, that is used in allocation policies.
Early mitigation efforts 
Efforts to delay address space exhaustion started with the recognition of the problem in the early 1990s, and include:
- Use of network address translation (NAT), in which many computers share one IP address, but which makes the computers behind the NAT unaddressable from the outside, breaking end-to-end connectivity
- Use of private network addressing
- Name-based virtual hosting of web sites
- Tighter control by regional Internet registries on the allocation of addresses to local Internet registries
- Network renumbering and subnetting to reclaim large blocks of address space allocated in the early days of the Internet, when the Internet used inefficient classful network addressing
Exhaustion dates and impact 
On 31 January 2011, the last two unreserved IANA /8 address blocks were allocated to APNIC according to RIR request procedures. This left five reserved but unallocated /8 blocks. In accord with ICANN policies, IANA proceeded to allocate one of those five /8s to each RIR, exhausting the IANA pool, at a ceremony and press conference on 3 February 2011.
The various legacy address blocks with administration historically split among the RIRs were distributed to the RIRs in February 2011.
APNIC was the first regional Internet Registry to run out of freely allocated IPv4 addresses, on 15 April 2011. This date marked the point where everybody who needed an IPv4 address could not be guaranteed to have one allocated. As a consequence of this exhaustion, end-to-end connectivity as required by specific applications will not be universally available on the Internet until IPv6 is fully implemented. However, IPv6 hosts cannot directly communicate with IPv4 hosts, and have to communicate using special gateway services. This means that general-purpose computers must still have IPv4 access, for example through NAT64, in addition to the new IPv6 address, which is more effort than just supporting IPv4 or IPv6. The demand for IPv6 is expected to ramp up to pervasiveness over three to four years.
In early 2011, only 16–26% of computers were latent IPv6 capable, while only 0.2% prefer IPv6 addressing with many using transition methods such as Teredo tunneling. About 0.15% of the top million websites are IPv6 accessible. Complicating matters, 0.027% to 0.12% of visitors cannot reach dual-stack sites, but a larger percentage (0.27%) cannot reach IPv4-only sites. IPv4 exhaustion mitigation technologies include IPv4 address sharing to access IPv4 content, IPv6 dual-stack implementation, protocol translation to access IPv4 and IPv6-addressed content, and bridging and tunneling to bypass single protocol routers. Early signs of accelerated IPv6 adoption after IANA exhaustion are evident.
Regional exhaustion 
APNIC was the first RIR to restrict allocations to 1024 addresses for each member due to its stock reaching critical levels of 1 /8 at 14 April 2011. The APNIC RIR is responsible for address allocation in the area where the Internet is growing the quickest with emerging markets like China and India.
Other RIRs are expected to exhaust within half a year to several years.
After IANA exhaustion, IPv4 address space requests are subject to additional restrictions at ARIN, but allocation policy is still largely unchanged. APNIC, LACNIC, and RIPE NCC are reserving the last obtained block for IPv6 transition, following special endgame set-aside policies.
Impact of APNIC RIR exhaustion and LIR exhaustion 
Systems that require inter-continental connectivity will have to deal with exhaustion mitigation already due to APNIC exhaustion. At APNIC, existing LIRs could apply for twelve months stock before exhaustion when they were using more than 80% of allocated space allocated to them. Since 15 April 2011, the date when APNIC reached its last /8 block, each (current or future) member will only be able to get one allocation of 1024 addresses (a /22 block) once. As the slope of the APNIC pool line on the "Geoff Huston's projection of the evolution of the IP pool for each RIR" chart to the right shows, the last /8 block would have been emptied within one month without this policy. By APNIC policy, each current or future member can receive only one /22 block from this last /8 (there are 16384 /22 blocks in the last /8 block). Since there are around 3000 current APNIC members, and around 300 new APNIC members each year, APNIC expects this last /8 block to last for many years.
The 1024 addresses in the /22 block can be used by APNIC members to supply NAT44 or NAT64 as a service on an IPv6 network. However at a new large ISP, 1024 IPv4 addresses might not be enough to provide IPv4 connectivity to all the customers due to the limited number of ports available per IPv4 address.
The Regional Internet Registries (RIR’s) for Asia (APNIC) and North America have a policy called the Inter-RIR IPv4 Address Transfer Policy, which allows IPv4 addresses to be transferred from North America to Asia. The ARIN policy was implemented on 31 July 2012.
IPv4 broker businesses have been established to facilitate these transfers.
Notable exhaustion advisories 
Estimates of the time of complete IPv4 address exhaustion varied widely in the early 2000s. In 2003, Paul Wilson (director of APNIC) stated that, based on then-current rates of deployment, the available space would last for one or two decades. In September 2005, a report by Cisco Systems suggested that the pool of available addresses would deplete in as little as 4 to 5 years. In the last year before exhaustion, IPv4 allocations were accelerating, resulting in exhaustion trending to earlier dates.
- On 21 May 2007, the American Registry for Internet Numbers (ARIN), the Anglo-American RIR, advised the Internet community that, due to the expected exhaustion in 2010, "migration to IPv6 numbering resources is necessary for any applications which require ongoing availability from ARIN of contiguous IP numbering resources". "Applications" include general connectivity between devices on the Internet, as some devices only have an IPv6 address allocated.
- On 20 June 2007, the Latin American and Caribbean Internet Addresses Registry (LACNIC), advised "preparing its regional networks for IPv6" by 1 January 2011, for the exhaustion of IPv4 addresses "in three years time".
- On 26 June 2007, the Asia-Pacific Network Information Centre (APNIC), the RIR for the Pacific and Asia, endorsed a statement by the Japan Network Information Center (JPNIC) that to continue the expansion and development of the Internet a move towards an IPv6-based Internet is advised. This, with an eye on the expected exhaustion around 2010, will create a great restriction on the Internet.
- On 26 October 2007, the Réseaux IP Européens Network Coordination Centre (RIPE NCC), the RIR for Europe, the Middle East, and parts of Central Asia, endorsed a statement by the RIPE community urging "the widespread deployment of IPv6 be made a high priority by all stakeholders".
- On 15 April 2009, ARIN sent a letter to all CEO/Executives of companies who have IPv4 addresses allocated informing them that ARIN expects the IPv4 space will be depleted within the next two years.
- In May 2009, the RIPE NCC launched IPv6ActNow.org to help explain "IPv6 in terms everyone can understand and providing a variety of useful information aimed at promoting the global adoption of IPv6".
- On 25 August 2009, ARIN announced a joint series event in the Caribbean region to push for the implementation of IPv6. ARIN reported at this time that less than 10.9% of IPv4 address space is remaining.
- World IPv6 Day was an event sponsored and organized by the Internet Society and several large content providers to test public IPv6 deployment. It started at 00:00 UTC on 8 June 2011 and ended at 23:59 the same day. The test primarily consisted of websites publishing AAAA records, allowing IPv6 capable hosts to connect to these sites using IPv6, and for misconfigured networks to be corrected.
- World IPv6 Launch occurred on 6 June 2012, following the success of World IPv6 Day a year earlier. It involved many more participants and had a more ambitious goal of permanently enabling IPv6 on participant organizations' networks. Wikimedia was one such organization that permanently enabled IPv6.
Post-exhaustion mitigation 
By 2008 policy planning for the end-game and post-exhaustion era was underway. Several proposals have been discussed to delay shortages of IPv4 addresses:
Reclamation of unused IPv4 space 
Before and during the time when classful network design was still used as allocation model, large blocks of IP addresses were allocated to some organizations. Since the use of Classless Inter-Domain Routing (CIDR) the Internet Assigned Numbers Authority (IANA) could potentially reclaim these ranges and reissue the addresses in smaller blocks. ARIN, RIPE NCC and APNIC have a transfer policy, such that addresses can get returned, with the purpose to be reassigned to a specific recipient. However, it can be expensive in terms of cost and time to renumber a large network, so these organizations are likely to object, with legal conflicts possible. However, even if all of these were reclaimed, it would only result in postponing the date of address exhaustion.
Similarly, IP address blocks have been allocated to entities that no longer exist and some allocated IP address blocks or large portions of them have never been used. No strict accounting of IP address allocations has been undertaken, and it would take a significant amount of effort to track down which addresses really are unused, as many are in use only on intranets.
Some address space previously reserved by IANA has been added to the available pool. There have been proposals to use the class E network range of IPv4 addresses, but many computer and router operating systems and firmware do not allow the use of these addresses. For this reason, the proposals have sought not to designate the class E space for public assignment, but instead propose to permit its private use for networks that require more address space than is currently available through RFC 1918.
Several organizations have returned large blocks of IP addresses. Notably, Stanford University relinquished their Class A IP block in 2000, making 16 million IP addresses available. Other organizations that have done so include the United States Department of Defense, BBN Technologies, and Interop.
Markets in IP addresses 
The creation of markets to buy and sell IPv4 addresses has been considered to be a solution to the problem of IPv4 scarcity and a means of redistribution. The primary benefits of an IPv4 address market are that it allows buyers to maintain undisrupted local network functionality. IPv6 adoption, while in progress, is currently still in early stages. It requires a significant investment of resources, and poses incompatibility issues with IPv4, as well as certain security and stability risks.
- According to some research, IPv6 traffic over 2011 has accounted for less than 0.3% of all the Internet traffic, regardless of the source. Also, very few ISPs currently deploy IPv6 to the consumer market, so it is not necessary to reach more consumers.
- The creation of a market in IPv4 addresses would only delay the practical exhaustion of the IPv4 address space for a relatively short time, since the public Internet is still growing.
- The concept of legal ownership of IP addresses as property is explicitly denied by ARIN and RIPE NCC policy documents and by the ARIN Registration Services Agreement, although ownership rights have been postulated based on a letter from the National Science Foundation General Counsel. NSF later indicated that the view was not official, and a statement from the Department of Commerce was subsequently issued indicating that "The USG participates in the development of and is supportive of the policies, processes, and procedures agreed upon by the Internet technical community through ARIN." 
- Ad-hoc trading in addresses could lead to fragmented patterns of routing that could expand the global routing table.
- Microsoft bought 666,624 IPv4 addresses from Nortel's liquidation sale for 7.5 million dollars in a deal brokered by Addrex. Before exhaustion, Microsoft could have obtained addresses from ARIN without charge, provided that, as per ARIN policy, Microsoft could present ARIN with a need for them. The success of this transfer was contingent on Microsoft successfully presenting ARIN with such a justification. The purchase provided Microsoft with a supply that was sufficient for their claimed needs for growth over the next 12 months, rather than for a 3-months' period as is normally requested from ARIN.
Transition mechanisms 
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As IPv4 addresses run out, some ISPs will not be able to provide globally routable IPv4 addresses to all their customers. Nevertheless those customers are likely to require access to servers that only have IPv4 addresses. Therefore, ISPs may have to provide a mechanism that allows those customers access to the IPv4 Internet. Several technologies have been developed for providing this IPv4 service over an IPv6 access network.
In ISP-level IPv4 NAT, ISPs may implement IPv4 network address translation within their networks and allocate private IPv4 addresses to customers. This approach has the advantage of allowing the customer to keep using their existing hardware. This has been successfully implemented in some countries, e.g., Russia, where many broadband providers use Carrier-grade NAT, and offer publicly routable IPv4 address at an additional cost.
However the allocation of private IPv4 addresses to customers may conflict with private IP allocations on the customer networks. Furthermore, very large ISPs may have to divide their network into subnets to allow them to reuse private IPv4 addresses, complicating network administration. There are also concerns that features of consumer-grade NAT such as DMZs, STUN, UPnP and application-level gateways might not be available at the ISP level. ISP-level NAT is likely to result in double NAT which is likely to further complicate the use of things like port forwarding used to run internet servers.
NAT64 translates IPv6 requests from clients to IPv4 requests. This avoids the need to provision any IPv4 addresses to clients and allows clients that only support IPv6 to access IPv4 resources. However this approach requires modifying DNS replies (DNS64) and cannot support IPv4-only client devices.
DS-Lite (Dual-Stack Light) uses tunnels from the customer premises equipment to a network address translator at the ISP. The consumer premise equipment encapsulates the IPv4 packets in an IPv6 wrapper and sends them to a host known as the AFTR element. The AFTR element de-encapsulates the packets and performs network address translation before sending them to the public Internet. The NAT in the AFTR uses the IPv6 address of the client in its NAT mapping table. This means that different clients can use the same private IPv4 addresses, therefore avoiding the need for allocating private IPv4 IP addresses to customers or using multiple NATs.
Address plus Port allows stateless sharing of public IP addresses based on TCP/UDP port numbers. Each node is allocated both an IPv4 address and a range of port numbers[clarification needed] to use. The technique avoids the need for stateful address translation mechanisms in the core of the network, thus leaving end users in control of their own address translation.
Long-term solution 
The deployment of IPv6 is the only available solution to the IPv4 address shortage. IPv6 is endorsed and implemented by all Internet technical standards bodies and network equipment vendors. It encompassed many design improvements, including the replacement of the 32-bit IPv4 address format with a 128-bit address for a capacity of about 3.4×1038 addresses. IPv6 has been in active production deployment since June 2006, after organized worldwide testing and evaluation in the 6bone project ceased.
See also 
- Smith, Lucie; Lipner, Ian (3 February 2011). "Free Pool of IPv4 Address Space Depleted". Number Resource Organization. Retrieved 3 February 2011.
- Available Pool of Unallocated IPv4 Internet Addresses Now Completely Emptied, Major Announcement Set on Dwindling Pool of Available IPv4 Internet Addresses
- ICANN,nanog mailing list. "Five /8s allocated to RIRs – no unallocated IPv4 unicast /8s remain".
- "RIPE NCC Begins to Allocate IPv4 Address Space From the Last /8". RIPE NCC.
- Huston, Geoff. "IPv4 Address Report, daily generated". Retrieved 16 January 2011.
- "Two /8s allocated to APNIC from IANA". APNIC. 1 February 2010. Retrieved 3 February 2011.
- "APNIC IPv4 Address Pool Reaches Final /8". APNIC. 15 April 2011. Retrieved 15 April 2011.
- RFC 4632
- Niall Richard Murphy, David Malone (2005). IPv6 network administration. O'Reilly Media, Inc. pp. xvii–xix. ISBN 0-596-00934-8.
- Mark Townsley (21 January 2011). "World IPv6 Day: Working Together Towards a New Internet Protocol".
- S.H. Gunderson (2008-10). "Global IPv6 Statistics – Measuring the current state of IPv6 for ordinary users" (PDF). Retrieved 10 November 2010.
- Ferguson, Tim (18 February 2007). "Broadband adoption passes halfway mark in U.S.". CNET News.com. Retrieved 10 November 2010.
- "Projections of the Number of Households and Families in the United States: 1995 to 2010" (PDF). 1996-04. Retrieved 10 November 2010.
- RFC 1918 Section 4
- IANA. "IANA IPv4 Address Space Registry". IANA IPv4 Address Space Registry. Retrieved 31 January 2011.
- Stephen Lawson (31 January 2011). "Address allocation kicks off IPv4 endgame". Computerworld.
- "Global Policy for the Allocation of the Remaining IPv4 Address Space". Retrieved 1 February 2011.
- "The IPv4 Depletion site » Blog Archive » Status of the various pool". Ipv4depletion.com. 3 December 2010. Retrieved 2 December 2011.
- "www.fix6.net". www.fix6.net. 24 November 2010. Retrieved 3 February 2011.
- "ISP Column - May 2011". Potaroo.net. Retrieved 2 December 2011.
- "IPv6 Measurements – A Compilation — RIPE Labs". Labs.ripe.net. Retrieved 2 December 2011.
- "IPV6 Test – Introductie". Ipv6test.max.nl. Retrieved 2 December 2011.
- "ISP Column – April 2010". Potaroo.net. Retrieved 2 December 2011.
- Carolyn Duffy Marsan (7 February 2011). "Suddenly everybody's selling IPv6". Network World.
- "APNIC's IPv4 pool usage". Apnic.net. Retrieved 2 December 2011.
- APNIC IPv4 Address Pool Reaches Final /8 [Apnic-announce], 15 April 2011
- "Registry Exhaustion Dates". Retrieved 4 April 2011.
- "information on ARIN website". Arin.net. Retrieved 3 February 2011.
- "APNIC – Policies for IPv4 address space management in the Asia Pacific region". Apnic.net. Retrieved 2 December 2011.
- "APNIC – Policies for IPv4 address space management in the Asia Pacific region". Apnic.net. Retrieved 2 December 2011.
- "APNIC – IPv4 exhaustion details". Apnic.net. 3 February 2011. Retrieved 2 December 2011.
- "IPv4 exhaustion details". APNIC.
- "No more addresses: Asia-Pacific region IPv4 well runs dry". Arstechnica. 15 April 2011. Retrieved 16 April 2011.
- Tomohiro Fujisaki (24 February 2011). "prop-095-v003: Inter-RIR IPv4 address transfer proposal". Retrieved 9 November 2011.
- "Draft Policy ARIN-2011-1: ARIN Inter-RIR Transfers". 14 October 2011. Retrieved 9 November 2011.
- Exec: No shortage of Net addresses By John Lui, CNETAsia
- Hain, Tony. "A Pragmatic Report on IPv4 Address Space Consumption". Retrieved 14 November 2007.
- "ARIN Board Advises Internet Community on Migration to IPv6" (Press release). American Registry for Internet Numbers (ARIN). 21 May 2007. Retrieved 1 July 2007.
- "LACNIC announces the imminent depletion of the IPv4 addresses" (Press release). Latin American and Caribbean Internet Addresses Registry (LACNIC). 21 June 2007. Retrieved 1 July 2007.
- "JPNIC releases statement on IPv4 consumption" (Press release). Asia-Pacific Network Information Centre (APNIC). 26 June 2007. Retrieved 1 July 2007.
- "About IPv4 address exhaustion in Internet Registries" (PDF) (Press release) (in Japanese). Japan Network Information Center (JPNIC). 19 June 2007. Retrieved 1 July 2007.
- "RIPE 55 – Meeting Report". RIPE NCC. 26 October 2007. Retrieved 2 February 2011.
- "Notice of Internet Protocol version 4 (IPv4) Address Depletion" (PDF). Retrieved 3 February 2011.
- White, Lauren (25 August 2009). "ARIN and Caribbean Telecommunications Union Host Premier Internet Community Meeting". Archived from the original on 27 August 2009. Retrieved 27 August 2009. "The global Internet community is playing a crucial role in the effort to raise awareness of IPv4 depletion and the plan to deploy IPv6, as only 10.9% of IPv4 address space currently remains."
- "Proposed Global Policy for the Allocation of the Remaining IPv4 Address Space". RIPE NCC. 3 March 2008. Retrieved 10 November 2010.
- "APNIC transfer policy". Apnic.net. 10 February 2010. Retrieved 3 February 2011.
- "ARIN transfer policy". Arin.net. Retrieved 3 February 2011.
- "Ripe Faq". Ripe.net. Retrieved 3 February 2011.
- Wilson, Paul; Michaelson, George; Huston, Geoff. "Redesignation of 240/4 from "Future Use" to "Limited Use for Large Private Internets" (expired draft)". Retrieved 5 April 2010.
- V. Fuller, E. Lear, D. Meyer (24 March 2008). "Reclassifying 240/4 as usable unicast address space (expired draft)". IETF. Retrieved 10 November 2010.
- "Address Classes". Windows 2000 Resource Kit. Microsoft. Retrieved 14 November 2007.
- van Beijnum, Iljitsch. "IPv4 Address Consumption". Retrieved 14 November 2007.
- "TCP/IP Overview". Cisco Systems, Inc. Retrieved 14 November 2007.
- Marsan, Carolyn. "Stanford move rekinds 'Net address debate". Network World. Retrieved 29 June 2010.
- "ARIN Recognizes Interop for Returning IPv4 Address Space". Arin.net. 20 October 2010. Retrieved 3 February 2011.
- Phil Lodico (15 September 2011). "Pssst! Rare IPv4 Addresses For Sale! Get Them While You Can!".
- KRISTINA BJORAN (27 July 2011). "The State of the Internet: IPv4 Won't Die".
- Steve Wexler (18 October 2011). "IPv6: Unstoppable Force Meets Immovable Object".
- David Braue (20 October 2011). "IPv6 will change network attack surface, albeit slowly: Huston".
- Elizabeth Harrin (22 September 2011). "IPv6 Will Cause Some Security Headaches".
- Ken Salchow (6 September 2011). "IPv6 migration: Do it for the right reasons".
- Mueller, Milton. "It's official: Legacy IPv4 address holders own their number blocks". Internet Governance Project. Retrieved 22 February 2013.
- Andrew, Dul. "Legacy IPv4 Address standing with USG". Retrieved 22 February 2013.
- Strickling, Lawrence. "United States Government’s Internet Protocol Numbering Principles". USG/NTIA. Retrieved 22 February 2013.
- Chloe Albanesius (25 March 2011). "Microsoft Spends $7.5M on 666K Nortel IPv4 Addresses". PC Magazine.
- Kevin Murphy (24 March 2011). "Microsoft spends $7.5 million on IP addresses". Domain Incite.
- "Resource Transfers: Returning Unneeded IPv4 Address Space". ARIN.
- Jaikumar Vijayan (25 March 2011). "IPv4 address transfers must meet policy, ARIN chief says".
- RFC 6333 - Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion
- Official current state of IPv4 /8 allocations, as maintained by IANA
- IPv6.com – Knowledge Center for Next Generation Internet IPv6
- ICANN recovers Large Block of Internet Addresses (188.8.131.52/8) 2008-02-10
- Global Policy Proposal for Remaining IPv4 Address Space – Background Report 2008-09-08
- potaroo.net: IPv4 Address Report with countdown
- RIR IPv4 status: APNIC RIPE