IPv4

1.0 Format

IPv4 is an identifier with a fixed length of 32-bit (232), which can represent about 4.3 billion different values1:

4 294 967 296

The human readable and easily memorable dot-decimal notation, which is also used in technical environments, groups the address into four parts where each consists of an unsigned decimal number with 8 bit:

255.255.255.255

An unsigned 8-bit decimal number can reflect 256 (28) different values (0-255). Since an address has four parts using such a number, its total length corresponds to 32 bit:

  •  8
  • 16
  • 24
  • 32
255.255.255.255

Working with 8-bit parts were the basis for a classful network design which was the standard in the 1980s. It divided the space into 8-bit groups (8, 16, and 24 bit) which were called Class A, B and C. This lead to inefficient assignments because a /8 subnet already had 16 777 216 addresses, while a /16 only had 65 536 addresses.

2.0 CIDR Table / Calculator

The classful network design was already replaced in the 1990s with a classless network design, which is called Classless Inter-Domain Routing (CIDR). It divides the space using 1-bit steps, as seen in the chart below:

    • Prefix
    • IP Addresses
    • /24s
    • /16s
    • /8s
    • Subnet Mask
    • /322
    • 1
    • 0
    • 0
    • 0
    • 0
    • 255.255.255.255
    • /31
    • 2
    • 0
    • 0
    • 0
    • 0
    • 255.255.255.254
    • /30
    • 4
    • 0
    • 0
    • 0
    • 0
    • 255.255.255.252
    • /29
    • 8
    • 1
    • 0
    • 0
    • 0
    • 255.255.255.248
    • /28
    • 16
    • 2
    • 0
    • 0
    • 0
    • 255.255.255.240
    • /27
    • 32
    • 4
    • 0
    • 0
    • 0
    • 255.255.255.224
    • /26
    • 64
    • 8
    • 0
    • 0
    • 0
    • 255.255.255.192
    • /25
    • 128
    • 16
    • 0
    • 0
    • 0
    • 255.255.255.128
    • /243
    • 256
    • 32
    • 1
    • 0
    • 0
    • 255.255.255.0
    • /23
    • 512
    • 64
    • 2
    • 0
    • 0
    • 255.255.254.0
    • /22
    • 1 024
    • 128
    • 4
    • 0
    • 0
    • 255.255.252.0
    • /21
    • 2 048
    • 256
    • 8
    • 0
    • 0
    • 255.255.248.0
    • /20
    • 4 096
    • 512
    • 16
    • 0
    • 0
    • 255.255.240.0
    • /19
    • 8 192
    • 1 024
    • 32
    • 0
    • 0
    • 255.255.224.0
    • /18
    • 16 384
    • 2 048
    • 64
    • 0
    • 0
    • 255.255.192.0
    • /17
    • 32 768
    • 4 096
    • 128
    • 0
    • 0
    • 255.255.128.0
    • /16
    • 65 536
    • 8 192
    • 256
    • 1
    • 0
    • 255.255.0.0
    • /15
    • 131 072
    • 16 384
    • 512
    • 2
    • 0
    • 255.254.0.0
    • /14
    • 262 144
    • 32 768
    • 1 024
    • 4
    • 0
    • 255.252.0.0
    • /13
    • 524 288
    • 65 536
    • 2 048
    • 8
    • 0
    • 255.248.0.0
    • /12
    • 1 048 576
    • 131 072
    • 4 096
    • 16
    • 0
    • 255.240.0.0
    • /11
    • 2 097 152
    • 262 144
    • 8 192
    • 32
    • 0
    • 255.224.0.0
    • /10
    • 4 194 304
    • 524 288
    • 16 384
    • 64
    • 0
    • 255.192.0.0
    • /9
    • 8 388 608
    • 1 048 576
    • 32 768
    • 128
    • 0
    • 255.128.0.0
    • /8
    • 16 777 216
    • 2 097 152
    • 65 536
    • 256
    • 1
    • 255.0.0.0
    • /7
    • 33 554 432
    • 4 194 304
    • 131 072
    • 512
    • 2
    • 254.0.0.0
    • /6
    • 67 108 864
    • 8 388 608
    • 262 144
    • 1 024
    • 4
    • 252.0.0.0
    • /5
    • 134 217 728
    • 16 777 216
    • 524 288
    • 2 048
    • 8
    • 248.0.0.0
    • /4
    • 268 435 456
    • 33 554 432
    • 1 048 576
    • 4 096
    • 16
    • 240.0.0.0
    • /3
    • 536 870 912
    • 67 108 864
    • 2 097 152
    • 8 192
    • 32
    • 224.0.0.0
    • /2
    • 1 073 741 824
    • 134 217 728
    • 4 194 304
    • 16 384
    • 64
    • 192.0.0.0
    • /1
    • 2 147 483 648
    • 268 435 456
    • 8 388 608
    • 32 768
    • 128
    • 128.0.0.0
    • /0
    • 4 294 967 296
    • 536 870 912
    • 16 777 216
    • 65 536
    • 256
    • 0.0.0.0

3.0 Exhaustion

While the IPv4 address space theoretically has more than 4.3 billion IP addresses, only 3.7 billion are available for public use because the rest is reserved for special purposes (e.g. 224/4 for multicast) or future use (240/4)4.

Due to this scarcity companies started to lease or buy IPv4 addresses from Local Internet Registries (LIRs).

August 1990 – Based on statistics, it was feared that an exhaustion of the pool could happen in 1994–1998.5

October 2002IANA had 73 /8 blocks left, which is equivalent to 1 224 736 768‬ (1.22 billion) IPv4 addresses.6

August 2004 – The available pool at IANA increased to 77 /8 blocks, which is equivalent to 1 291 845 632‬ (1.29 billion) IPv4 addresses.7 This is related to the blocks 173/8–187/8 which were previously allocated to Various Registries in 1993, but seems to be returned to IANA in April 2003.

May 2008 – IANA had 40 /8 blocks left, which is equivalent to 671 088 640 (671 million) IPv4 addresses.8

February 2011 – IANA assigned the last five available /8 blocks equally to all five RIRs9. AFRINIC received 102/8, APNIC 103/8, ARIN 104/8, LACNIC 179/8 and RIPE NCC 185/810.

April 2011 – APNIC reached the last /8 block (103/8).11

March 2012 – RIPE NCC reached the last /8 block (185/8) and moved to an only one /22 per LIR policy.12

May 2012 – The ICANN board ratified the Global Policy for Post Exhaustion IPv4 Allocation Mechanisms by the IANA which regulates the distribution of address space returned to IANA: […] its size is not expected to be so large but substantial enough to be able to consider an additional minimum allocation for APNIC members.13

June 2014 – LACNIC reported its (relatively) exhaustion because they reached the last /10 block14, so the further allocation rate has been greatly reduced15. Since there is still space available which will probably last until May 202016, new members were still eligible to receive their initial /24 or /22 allocation in 2019.

July 2015 – RIPE NCC changed its policy in order to block transfers of allocations made to members for a period of 24 months. The reason for that change was a proposal which stated that speculators had opened new LIR accounts only to sell the initial /22 allocation given according to the one /22 per LIR policy.17

September 2015 – ARIN finally allocated all available IPv4 addresses.18 Further requests can only be fulfilled by using recovered space (e.g. revoked space due to non-payments) and are handled through a waiting list. ARIN was the first RIR who became (absolutely) exhausted.

November 2015 – The RIPE NCC Executive Board decided to temporarily restrict the ability for members to create multiple LIR accounts because it was considered as not conform to the policy.19

May 2016 – The members of the RIPE NCC General Meeting voted for a resolution which removed the previously applied restrictions which temporarily blocked members from creating multiple LIR accounts.20 Prior to this, some organizations already circumvented the restrictions by simply creating new business entities.21

January 2017 – AFRINIC entered IPv4 Exhaustion Phase 1/2.22

April 2018 – RIPE NCC allocated the last addresses from the last 185/823. Since then they have started to allocate from over nine million recovered IPv4 addresses (about a IPv4 /9).

July 2019 – AFRINIC had about 5.15 million addresses left24 in 2019, which is equivalent to IPv4 /10 + /12 or 5 034 x IPv4 /22. AFRINIC is the RIR with the most IPv4 addresses left in 201925.

4.0 Current Distribution

4.1 Reserved Space

According to the IANA IPv4 Special-Purpose Address Registry (last updated in July 2017) in total there are 23 allocations with 324 273 934 IPv4 addresses reserved for special purposes or future use at the moment.

Not included in this list is 224/4 which is reserved for multicasting. This increases the amount of total reserved IPv4 addresses to 592 709 390 which lefts an amount of 3 702 257 906 (3.7 billion) available IPv4 addresses.

    • Address Block(s)
    • Addresses
    • Name
    • Allocation Date
    • 0.0.0.0/8
    • 16 777 216
    • "This host on this network"
    • Sep 1981
    • 10.0.0.0/8
    • 16 777 216
    • Private-Use
    • Feb 1996
    • 100.64.0.0/10
    • 4 194 304
    • Shared Address Space
    • Apr 2012
    • 127.0.0.0/8
    • 16 777 216
    • Loopback
    • Sep 1981
    • 169.254.0.0/16
    • 65 536
    • Link Local
    • May 2005
    • 172.16.0.0/12
    • 1 048 576
    • Private-Use
    • Feb 1996
    • 192.0.0.0/24
    • 256
    • IETF Protocol Assignments
    • Jan 2010
    • 192.0.0.0/29
    • 8
    • IPv4 Service Continuity Prefix
    • Jun 2011
    • 192.0.0.8/32
    • 1
    • IPv4 dummy address
    • Mar 2015
    • 192.0.0.9/32
    • 1
    • Port Control Protocol Anycast
    • Oct 2015
    • 192.0.0.10/32
    • 1
    • Traversal Using Relays around NAT Anycast
    • Feb 2017
    • 192.0.0.170/32
      192.0.0.171/32
    • 2
    • NAT64/DNS64 Discovery
    • Feb 2013
    • 192.0.2.0/24
    • 256
    • Documentation (TEST-NET-1)
    • Jan 2010
    • 192.31.196.0/24
    • 256
    • AS112-v4
    • Dec 2014
    • 192.52.193.0/24
    • 256
    • AMT
    • Dec 2014
    • 192.88.99.0/24
    • 256
    • Deprecated (6to4 Relay Anycast)
    • Jun 200126
    • 192.168.0.0/16
    • 65 536
    • Private-Use
    • Feb 1996
    • 192.175.48.0/24
    • 256
    • Direct Delegation AS112 Service
    • Jan 1996
    • 198.18.0.0/15
    • 131 072
    • Benchmarking
    • Mar 1999
    • 198.51.100.0/24
    • 256
    • Documentation (TEST-NET-2)
    • Jan 2010
    • 203.0.113.0/24
    • 256
    • Documentation (TEST-NET-3)
    • Jan 2010
    • 240.0.0.0/4
    • 268 435 456
    • Reserved
    • Aug 1989
    • 255.255.255.255/32
    • 1
    • Limited Broadcast
    • Oct 1984

4.2 Allocated Space

According to the IANA IPv4 Address Space Registry (last updated in July 2019), excluding reserved /8s such as 127/8 (loopback) or 240/4 (future use), there have been made 221 /8 allocations in total, primarily to RIRs, but due to historical reasons also to a few companies. This is an amount of 3 707 764 736 total allocated IPv4 addresses.

However, from this amount deducted are 5 506 830 (5.5 million) reserved addresses which are part from reserved allocations smaller than /8, but still part of the allocated /8s (see Reserved Space). For example, 100.64.0.0/10 and multiple /24s of 192.0.0.0/8 are reserved, while the larger blocks 100/8 and 192/8 are still allocated to ARIN.

    • Designation / Allocations made
    • Allocations / IPs
    • Prefix(es)
    • IANA - Local Identification
      1981
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 000/8
    • APNIC
      1993–2011
    • 45 allocations
      754 974 720
      ≅ 17.58%
    • 001/8  014/8  027/8  036/8  039/8  042/8  049/8  058/8  059/8  060/8  061/8  101/8  103/8  106/8  110/8  111/8  112/8  113/8  114/8  115/8  116/8  117/8  118/8  119/8  120/8  121/8  122/8  123/8  124/8  125/8  126/8  175/8  180/8  182/8  183/8  202/8  203/8  210/8  211/8  218/8  219/8  220/8  221/8  222/8  223/8
    • RIPE NCC
      1993–2011
    • 35 allocations
      587 202 560
      ≅ 13.67%
    • 002/8  005/8  031/8  037/8  046/8  062/8  077/8  078/8  079/8  080/8  081/8  082/8  083/8  084/8  085/8  086/8  087/8  088/8  089/8  090/8  091/8  092/8  093/8  094/8  095/8  109/8  176/8  178/8  185/8  193/8  194/8  195/8  212/8  213/8  217/8
    • Administered by ARIN
      1991–1995
    • 57 allocations
      956 301 312
      ≅ 22.27%
    • 003/8  004/8  007/8  008/8  009/8  013/8  015/8  016/8  018/8  020/8  032/8  034/8  035/8  040/8  044/8  045/8  047/8  052/8  054/8  128/8  129/8  130/8  131/8  132/8  134/8  135/8  136/8  137/8  138/8  139/8  140/8  142/8  143/8  144/8  146/8  147/8  148/8  149/8  152/8  155/8  156/8  157/8  158/8  159/8  160/8  161/8  162/8  164/8  165/8  166/8  167/8  168/8  169/8  170/8  172/8  192/8  198/8
    • Army Information Systems Center
      1994
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 006/8
    • IANA - Private Use
      1995
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 010/8
    • DoD Intel Information Systems
      1993
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 011/8
    • AT&T Bell Laboratories
      1995
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 012/8
    • Apple Computer Inc.
      1992
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 017/8
    • Ford Motor Company
      1995
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 019/8
    • DDN-RVN
      1991
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 021/8
    • Defense Information Systems Agency
      1991–1995
    • 4 allocations
      67 108 864
      ≅ 1.56%
    • 022/8  026/8  029/8  030/8
    • ARIN
      1993–2011
    • 36 allocations
      603 979 776
      ≅ 14.06%
    • 023/8  024/8  050/8  063/8  064/8  065/8  066/8  067/8  068/8  069/8  070/8  071/8  072/8  073/8  074/8  075/8  076/8  096/8  097/8  098/8  099/8  100/8  104/8  107/8  108/8  173/8  174/8  184/8  199/8  204/8  205/8  206/8  207/8  208/8  209/8  216/8
    • Administered by RIPE NCC
      1993–1995
    • 6 allocations
      100 663 296
      ≅ 2.34%
    • 025/8  051/8  141/8  145/8  151/8  188/8
    • DSI-North
      1992
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 028/8
    • DLA Systems Automation Center
      1991
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 033/8
    • PSINet, Inc.
      1994
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 038/8
    • AFRINIC
      2005–2011
    • 4 allocations
      67 108 864
      ≅ 1.56%
    • 041/8  102/8  105/8  197/8
    • Administered by APNIC
      1991–1997
    • 6 allocations
      100 663 296
      ≅ 2.34%
    • 043/8  133/8  150/8  153/8  163/8  171/8
    • Prudential Securities Inc.
      1995
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 048/8
    • Daimler AG
      1993
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 053/8
    • DoD Network Information Center
      1995
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 055/8
    • US Postal Service
      1994
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 056/8
    • Societe Internationale de Telecommunications Aeronautiques S.C.R.L.
      1995
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 057/8
    • IANA - Loopback
      1981
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 127/8
    • Administered by AFRINIC
      1993
    • 2 allocations
      33 554 432
      ≅ 0.78%
    • 154/8  196/8
    • LACNIC
      1995–2011
    • 9 allocations
      150 994 944
      ≅ 3.52%
    • 177/8  179/8  181/8  186/8  187/8  189/8  190/8  200/8  201/8
    • Administered by LACNIC
      1993
    • 1 allocation
      16 777 216
      ≅ 0.39%
    • 191/8
    • US-DOD
      1998
    • 2 allocations
      33 554 432
      ≅ 0.78%
    • 214/8  215/8
    • Multicast
      1981
    • 16 allocations
      268 435 456
      ≅ 6.25%
    • 224/8  225/8  226/8  227/8  228/8  229/8  230/8  231/8  232/8  233/8  234/8  235/8  236/8  237/8  238/8  239/8
    • Future use
      1981
    • 16 allocations
      268 435 456
      ≅ 6.25%
    • 240/8  241/8  242/8  243/8  244/8  245/8  246/8  247/8  248/8  249/8  250/8  251/8  252/8  253/8  254/8  255/8