Internet Protocol version 4 (IPv4) is the version of internet protocol which defines IP addresses in a 32-bit format. It has been the general standard to deliver information between devices connected to the Internet. The last blocks of IPv4 addresses were allocated by IANA to the Regional Internet Registries in February, 2011. While IPv4 supply has dwindled over the years, supplies have reached a critical point in most regions of the world. As of 24 September 2015, ARIN (American Registry for Internet Numbers) reported that it had run out of its free pool of IPv4 addresses. While the dwindling addresses will not impact Internet users too greatly, ISPs have increasingly felt the pressure to modify their technologies to suit IPv6. 
IPv4 was invented in the 1970s as the first major version of internet protocol. IPV4 allows numbers to map to physical devices and build a logical method for traffic to route from one number to another. Since its first introduction to the public in 1981, IPv4 became the foundation of the Internet and many other enterprise networks worldwide.
The total number of unique IP addresses available under the IPv4 format is limited to approximately 4,294,967,296 billion, however it is not enough to cope with projected Internet demand due to the increasing reliance on personal computers, wireless communication devices and radio frequency identification (RFID) tags, etc. The Internet Engineering Task Force predicted that the available IP addresses will be depleted as early as 2011.
IPv4 Header Format
An IPv4 header contains the following fields:
- Version- The IP version number, 4.
- Length - The length of the datagram header in 32-bit words.
- Type of service- Contains five subfields that specify the precedence, delay, throughput, reliability, and cost desired for a packet. (The Internet does not guarantee this request.) This field is not widely used on the Internet.
- Total length- The length of the datagram in bytes including the header, options, and the appended transport protocol segment or packet.
- Identification- An integer that identifies the datagram.
- Flags-Controls datagram fragmentation together with the identification field. The flags indicate whether the datagram may be fragmented, whether the datagram is fragmented, and whether the current fragment is the final one.
- Fragment offset-The relative position of this fragment measured from the beginning of the original datagram in units of 8 bytes.
- Time to live- How many routers a datagram can pass through. Each router decrements this value by 1 until it reaches 0 when the datagram is discarded. This keeps misrouted datagrams from remaining on the Internet forever.
- Protocol- The high-level protocol type.
- Header checksum- A number that is computed to ensure the integrity of the header values.
- Source address- The 32-bit IPv4 address of the sending host.
- Destination address- The 32-bit IPv4 address of the receiving host.
- Options- A list of optional specifications for security restrictions, route recording, and source routing. Not every datagram specifies an options field.
- Padding- Null bytes which are added to make the header length an integral multiple of 32 bytes as required by the header length field.
IPv4 Address Space & Notation
IP addresses are 32-bit binary numbers used as addresses in the IPv4 protocol. The three main types of IPv4 addresses are Public IP addresses, Private IP Networks and Global or Specialized IP Addresses.
IPv4 addresses uniquely identify every host computer connected to the Internet, which is typically represented as four decimal numbers separated by dots, for example: 126.96.36.199. A TCP/IP network routes a packet according to the destination IP address, an address provided by the IP protocol on the sending host. The network address (the part of the address that identifies a users network) and the host address (the part that identifies an individual host on a users network) must all fit into the 32-bit number. The number of hosts an Internet user can configure for the network decreases as the length of the portion occupied by the network address increases. The Network Information Centers or NICs (including the original InterNIC) assign network addresses for the IPv4-based Internet.
IPv4 Address Allocation
- AfriNIC - Africa, portions of the Indian Ocean
- APNIC - Most of Asia, Oceania
- ARIN - Canada, many Caribbean and North Atlantic islands, and the United States
- LACNIC - Latin America, portions of the Caribbean
- RIPE NCC - Europe, the Middle East, Central Asia
The Regional Internet Registries are responsible for the assignment of IPv4 addresses to operators and users of the Internet within their regions.
In February, 2011, the Number Resource Organization announced that the free pool of available IPv4 addresses had been fully depleted. On January 31, IANA had allocated two blocks of IPv4 address space to APNIC. The action triggered a global policy to allocate the remaining IANA pool equally between the five Regional Internet Registries. IANA generally assigns IPv4 addresses to the RIRs in blocks that equate to 1/256th of the entire IPv4 address space. Each block is referred to as a “/8″ or “slash-8″.  In 2009, ICANN ratified a global policy agreed upon by the RIRs stipulating that when the IANA IPv4 free pool reached five remaining /8 blocks, It will be simultaneously and equally distributed to the five RIRs.
The last five blocks of IPv4 addresses were allocated to the five RIRs during a ceremony and meeting of the RIRs which was held in Miami, Florida. According to ICANN, the allocation of the final IPv4 addresses is analogous to the last crates of a product leaving a manufacturing warehouse and going to the regional stores or distributions centers, where they can still be distributed to the public. Once they are gone, the supply is exhausted. In this case, the RIRs will distribute the last IPv4 addresses to Internet Service Providers, universities, governments, telecommunications companies and other enterprises. Raúl Echeberría, Chairman of the NRO emphasized, "It’s only a matter of time before the RIRs and Internet Service Providers (ISPs) must start denying requests for IPv4 address space". He also pointed out that, "Deploying IPv6 is now a requirement, not an option."
By April 2011, APNIC, Asia's RIR, had depleted all but a small subsection of its IPv4 allocation. These remaining addresses were placed on hold for start-up network operators. APNIC was the first RIR to deplete its free pool of Internet address space. Geoff Huston, adjunct research fellow at the Centre for Advanced Internet Architectures at Swinburne University of Technology, estimates that Europe's RIR, RIPE NCC, will run out of its remaining IPv4 address space on July 22, 2012. ARIN, North America's RIR, is expected to run out in 2013.
- ARIN IPV4 FREE POOL REACHES ZERO Retrieved 2 December 2015.
- North America Just Ran Out Retrieved 28 Sep 2015.
- IPv4 Header Format
- IPv4 Header Format
- Asia out of IPv4 addresses, networkworld.com
- 5 Major Changes Facing the Internet in 2012