What is IPv4?
A comprehensive guide to Internet Protocol version 4 — the addressing system that powers the modern internet. Understand how IPv4 works, why addresses are running out, and what it means for your business.
What is IPv4?
IPv4, or Internet Protocol version 4, is the fourth revision of the Internet Protocol, originally defined in RFC 791, and the first version to be widely deployed. It is the foundational protocol that enables devices to communicate over the internet by assigning each one a unique numerical address. Every time you visit a website, send an email, or stream a video, IPv4 is working behind the scenes to route data packets between your device and the destination server.
IPv4 uses a 32-bit address scheme, which allows for approximately 4.3 billion unique addresses (2³² = 4,294,967,296). While this seemed like an enormous number when the protocol was designed in the early 1980s, the explosive growth of internet-connected devices — from smartphones and laptops to IoT sensors and cloud servers — has long since exhausted the available pool. The IANA IPv4 address space registry shows that all /8 blocks have been allocated to RIRs.
Despite the development of its successor, IPv6, which offers a virtually unlimited address space, IPv4 remains the dominant protocol on the internet today. Learn more in our what is IPv6 guide. The vast majority of websites, networks, and services still rely on IPv4, making these addresses a critical and increasingly scarce digital resource.
IPv4 Address Format
An IPv4 address is a 32-bit number typically written in dotted-decimal notation, consisting of four octets separated by periods. Each octet represents 8 bits and can hold a value between 0 and 255. For example, the address 192.168.1.1 is one of the most commonly recognized IPv4 addresses, used as a default gateway in many home routers.
In binary, each octet is represented by 8 binary digits (bits). The address 192.168.1.1 translates to 11000000.10101000.00000001.00000001 in binary. The 32-bit address is divided into two logical parts: the network portion (which identifies the network) and the host portion (which identifies the specific device on that network). The boundary between these two parts is defined by the subnet mask.
Octet Structure
Each IPv4 address consists of four 8-bit octets (bytes), giving a total of 32 bits. Each octet can represent values from 0 to 255 in decimal notation.
Binary Representation
Under the hood, IPv4 addresses are 32-bit binary numbers. For example, 10.0.0.1 is 00001010.00000000.00000000.00000001 in binary.
Dotted Decimal
The standard human-readable format uses four decimal numbers separated by dots (e.g., 172.16.254.1). This notation makes addresses easier to read and remember.
Network vs Host
The subnet mask divides the address into a network portion (identifying the network) and a host portion (identifying the device). For example, in a /24 network, the first 24 bits identify the network.
IPv4 Address Classes
IPv4 addresses were originally organized into five classes (A through E), each designed for different network sizes. This classful addressing scheme determined how the 32-bit address was split between the network and host portions. Although classful addressing has largely been replaced by CIDR (Classless Inter-Domain Routing), understanding the classes remains important for networking fundamentals. See our detailed IP address classes guide for more.
Class A networks (1.0.0.0 – 126.255.255.255) use the first octet for the network ID and the remaining three for hosts, supporting up to 16.7 million hosts per network. Class B (128.0.0.0 – 191.255.255.255) uses two octets each for network and host, supporting 65,534 hosts. Class C (192.0.0.0 – 223.255.255.255) uses three octets for the network and one for hosts, supporting 254 hosts per network.
Class A
Range: 1.0.0.0 – 126.255.255.255. Designed for very large networks with up to 16.7 million hosts. Only 128 Class A networks exist, assigned to major organizations and ISPs.
Class B
Range: 128.0.0.0 – 191.255.255.255. Suited for medium to large organizations with up to 65,534 hosts per network. There are 16,384 possible Class B networks.
Class C
Range: 192.0.0.0 – 223.255.255.255. Designed for smaller networks with up to 254 hosts. Class C is the most common class, with over 2 million possible networks.
Class D & E
Class D (224.0.0.0 – 239.255.255.255) is reserved for multicast groups. Class E (240.0.0.0 – 255.255.255.255) is reserved for experimental and future use.
Private vs Public IPv4 Addresses
Not all IPv4 addresses are created equal. Public IPv4 addresses are globally unique and routable on the internet — they are what web servers, email services, and any internet-facing system uses to communicate. These are the addresses that have become scarce and valuable in the IPv4 market.
Private IPv4 addresses, defined by RFC 1918, are reserved for use within local networks and are not routable on the public internet. The three private address ranges are: 10.0.0.0 – 10.255.255.255 (a single Class A block), 172.16.0.0 – 172.31.255.255 (16 Class B blocks), and 192.168.0.0 – 192.168.255.255 (256 Class C blocks). These addresses can be reused by any organization within their internal networks.
Network Address Translation (NAT) bridges the gap between private and public addresses. NAT allows multiple devices on a private network to share a single public IPv4 address when accessing the internet. While NAT has been instrumental in extending the life of the IPv4 address space, it adds complexity, can break certain applications, and does not eliminate the underlying need for public addresses.
IPv4 Address Exhaustion
IPv4 address exhaustion refers to the depletion of the available pool of unallocated IPv4 addresses. The Internet Assigned Numbers Authority (IANA) allocated its last blocks of IPv4 addresses to the Regional Internet Registries (RIRs) on February 3, 2011, marking a historic milestone in the history of the internet.
Following IANA exhaustion, each RIR gradually depleted its own free pool. APNIC (Asia-Pacific) ran out in April 2011, RIPE NCC (Europe) in September 2012, LACNIC (Latin America) in June 2014, ARIN (North America) in September 2015, and AFRINIC (Africa) in January 2020. Most RIRs now operate waiting lists for small allocations (/24 blocks), but the supply is extremely limited.
The exhaustion was driven by the explosive growth of internet-connected devices, cloud computing, mobile networks, and the Internet of Things (IoT). With only 4.3 billion possible addresses and billions of devices needing connectivity, the math simply could not work. This scarcity has given rise to a thriving secondary market where organizations buy, sell, and lease IPv4 addresses.
The IPv4 Address Market
The IPv4 address market emerged as a direct consequence of address exhaustion. Since new IPv4 addresses can no longer be obtained from RIRs through traditional allocation, organizations that need public IPv4 space must acquire it from existing holders through regulated transfers. This secondary market has grown into a mature, transparent marketplace with established pricing, professional brokers, and clear transfer procedures governed by each RIR.
IPv4 brokers like IPv4Center.com facilitate these transactions by connecting buyers and sellers, performing due diligence (including blacklist screening across 300+ databases), managing the RIR transfer process, and providing escrow payment protection. Prices vary by block size and RIR region, with per-IP costs typically ranging from $19 to $35 as of 2026. Organizations can also choose to lease IPv4 addresses for lower upfront costs, making IP space accessible regardless of budget.
How IPv4 Addresses Work
An IPv4 address is a 32-bit numerical identifier assigned to every device connected to a network using the Internet Protocol version 4. Written in dotted-decimal notation (e.g., 192.168.1.1), each of the four octets represents 8 bits ranging from 0 to 255, providing a total address space of approximately 4.3 billion unique addresses (2^32 = 4,294,967,296). Every device that communicates directly on the public internet must have a unique public IPv4 address — this uniqueness is what makes routing possible across the global network.
When a device sends data over the internet, the information is broken into packets, each stamped with the source and destination IPv4 addresses in its header. Routers along the path examine the destination address and consult their routing tables to forward each packet toward its destination. This process, defined in RFC 791 (published in 1981), is the fundamental mechanism that enables global internet communication. The Time to Live (TTL) field in the IPv4 header prevents packets from circulating indefinitely by decrementing at each hop and discarding the packet when it reaches zero.
IPv4 operates at Layer 3 (the Network Layer) of the OSI model and is a connectionless, best-effort delivery protocol — meaning it does not guarantee packet delivery, ordering, or duplicate protection. Higher-layer protocols like TCP (Transmission Control Protocol) provide reliability on top of IPv4. The protocol also supports fragmentation, allowing large packets to be broken into smaller fragments when they encounter a network segment with a smaller Maximum Transmission Unit (MTU), then reassembled at the destination.
32-Bit Address Space
Four octets of 8 bits each, totaling 32 bits. This yields 2^32 = 4,294,967,296 possible addresses, a finite pool now fully allocated.
Dotted-Decimal Notation
Human-readable format using four decimal numbers (0–255) separated by dots. Example: 192.168.1.1 represents binary 11000000.10101000.00000001.00000001.
Packet Routing
Each packet carries source and destination IPv4 addresses. Routers use these addresses to forward packets hop-by-hop toward the destination network.
Best-Effort Delivery
IPv4 itself does not guarantee delivery or ordering. Protocols like TCP add reliability, while UDP trades reliability for speed in real-time applications.
IPv4 Address Assignment and Allocation
IPv4 address allocation follows a hierarchical delegation model governed by the Internet Assigned Numbers Authority (IANA). IANA delegates large blocks of address space to five Regional Internet Registries (RIRs): ARIN (North America), RIPE NCC (Europe, Middle East, Central Asia), APNIC (Asia Pacific), LACNIC (Latin America and Caribbean), and AFRINIC (Africa). Each RIR then allocates smaller blocks to Internet Service Providers (ISPs) and Local Internet Registries (LIRs), which in turn assign addresses to end-user organizations and individual customers.
IANA exhausted its free pool of IPv4 addresses on February 3, 2011, when the last five /8 blocks were distributed — one to each RIR under a pre-agreed global policy. Since that date, new IPv4 addresses are no longer available from IANA. Each RIR subsequently depleted its own reserves: APNIC in April 2011, RIPE NCC in September 2012, LACNIC in June 2014, ARIN in September 2015, and AFRINIC in January 2020. Some RIRs maintain small reserved pools for critical infrastructure or transition technologies, but general-purpose allocation is no longer possible.
Today, organizations needing IPv4 address space must acquire it through the secondary transfer market, where existing address holders sell or lease their unused allocations. Each RIR maintains its own transfer policy: ARIN requires demonstrated need, RIPE NCC allows transfers without a needs assessment, and APNIC requires a 24-month demonstrated need. IPv4Center.com facilitates transfers across all five RIR regions, handling due diligence, escrow, and the complete administrative transfer process. See our price tracker for current market rates.
Frequently Asked Questions
Common questions about IPv4 and internet addressing.
IPv4 uses 32-bit addresses (about 4.3 billion unique addresses), while IPv6 uses 128-bit addresses (about 340 undecillion). IPv6 was designed to solve IPv4 exhaustion and includes improvements like simplified headers, built-in IPsec, and no need for NAT. However, IPv4 remains dominant and the two protocols are not directly compatible.
The 4.3 billion addresses provided by IPv4's 32-bit scheme were not enough to keep up with the explosion of internet-connected devices, cloud services, and mobile networks. IANA distributed its last IPv4 blocks in 2011, and all five RIRs have since exhausted their free pools.
Yes, but not through traditional RIR allocation. You can acquire IPv4 addresses through the secondary market by purchasing or leasing them from existing holders. Brokers like IPv4Center.com facilitate these transfers with escrow protection and full RIR transfer management.
Private IPv4 addresses (defined by RFC 1918) are reserved for internal network use and are not routable on the public internet. The ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Devices using private addresses access the internet through NAT (Network Address Translation).
As of 2026, IPv4 addresses cost approximately $19–$35 per IP depending on block size and RIR region. Larger blocks offer lower per-IP pricing. RIPE NCC blocks tend to command premium prices due to high European demand. Check our pricing page for current rates.
A full transition is unlikely in the near term. While IPv6 adoption is growing, the vast majority of internet infrastructure still relies on IPv4. Most experts expect both protocols to coexist for many years, which means IPv4 addresses will continue to hold significant value.