IPv4

Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol and the first to be widely deployed. It is a connectionless protocol operating at the network layer of the OSI model, responsible for addressing hosts and routing packets across networks.

IPv4 uses 32-bit addresses, providing approximately 4.3 billion unique addresses (2^32). Despite its limitations in address space, IPv4 remains the dominant protocol on the internet, powering the majority of traffic through mechanisms like NAT and CIDR that extend its usability.

IPv4 was defined in RFC 791 in 1981 by the U.S. Department of Defense as part of the early internet development. It replaced earlier protocols and became the standard for ARPANET, transitioning to the global internet in the 1980s.

Classful addressing was introduced early on, dividing the space into classes A–E. The 1990s saw explosive growth, leading to the adoption of Classless Inter-Domain Routing (CIDR) in 1993 to slow exhaustion. Regional Internet Registries (RIRs) formed to manage allocations.

By 2011, IANA exhausted its free pool, and RIRs followed in the 2010s. IPv4 continues in widespread use alongside IPv6 transition mechanisms.

IPv4 packets contain a header (minimum 20 bytes) with source and destination addresses, along with fields for fragmentation, TTL, protocol, and checksum.

Routing relies on longest-prefix matching in routing tables. Packets are forwarded hop-by-hop until reaching the destination or discarded if TTL expires.

IPv4 Header (simplified):
Version | IHL | Type of Service | Total Length
Identification | Flags | Fragment Offset
Time to Live | Protocol | Header Checksum
Source Address (32 bits)
Destination Address (32 bits)
Options (if any) | Padding

Fragmentation allows large packets to be split for transmission, reassembled at the destination.

IPv4 addresses are 32 bits, written in dotted-decimal notation (e.g., 192.168.1.1).

Originally classful:

• Class A: 0.0.0.0 – 127.255.255.255 (large networks)
• Class B: 128.0.0.0 – 191.255.255.255
• Class C: 192.0.0.0 – 223.255.255.255 (small networks)
• Class D: 224.0.0.0 – 239.255.255.255 (multicast)
• Class E: 240.0.0.0 – 255.255.255.255 (reserved)

CIDR superseded classes, allowing variable-length subnet masks (VLSM).

Key reserved blocks:

• 0.0.0.0/8: This network
• 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16: Private (RFC 1918)
• 127.0.0.0/8: Loopback
• 169.254.0.0/16: Link-local (APIPA)
• 198.18.0.0/15: Benchmarking
• 255.255.255.255: Broadcast

For reverse DNS lookups on IPv4 addresses (PTR records), tools like Reverse IP Lookup can reveal associated hostnames and organizational details.

Subnetting divides networks into smaller segments using masks (e.g., /24 = 255.255.255.0). CIDR enables aggregation and efficient allocation.

Modern routing uses prefix-length notation. NAT translates private addresses to public, conserving space.

IPv4 addresses power:

• Public internet routing
• Private internal networks
• Cloud and virtual machine addressing
• IoT and embedded devices
• Legacy systems and compatibility layers

Determining the approximate geographic location of an IPv4 address is useful for analytics and security – My Location provides real-time geolocation based on your current public IP.

Basic connectivity testing remains essential; a Ping Online can verify reachability and latency to any IPv4 address.

Primary issues:

• Address exhaustion (free pool depleted)
• Security vulnerabilities (no built-in encryption/IPsec optional)
• Fragmentation overhead
• Complexity with NAT breaking end-to-end connectivity

Many addresses end up on blacklists due to abuse (spam, malware). Checking if an IPv4 address is listed can prevent deliverability issues – use IP Blacklist Checker for real-time reputation scans.

By 2026, IPv4 coexists with IPv6 through dual-stack, tunneling (6to4, Teredo), and carrier-grade NAT (CGNAT). Markets trade IPv4 addresses due to scarcity.

Transition mechanisms like NAT64/DNS64 enable IPv6-only clients to reach IPv4 resources. Many new deployments prioritize IPv6, but IPv4 persists in legacy and constrained environments.

Internet Protocol version 4 has been the workhorse of the internet for over four decades, providing robust addressing and routing despite its limited 32-bit space. Techniques like CIDR, NAT, and private addressing extended its life far beyond original expectations. While IPv6 gradually takes over, IPv4 remains critical for compatibility and continues to carry the majority of global traffic through careful management and conservation strategies.

• RFC 791 – Internet Protocol
• RFC 1918 – Address Allocation for Private Internets
• RFC 4632 – Classless Inter-Domain Routing (CIDR)
• ARIN/RIR IPv4 Depletion Reports

Information compiled from IETF RFCs, RIR documentation, networking textbooks (Kurose & Ross), industry reports (APNIC, ARIN), and technical resources up to 2026.

• IPv4 on Wikipedia
• IANA IPv4 Special Registry
• ARIN IPv4 Resources
• RFC 791 – IPv4 Specification