What is IPv4?
IPv4 is the core Internet Protocol using 32-bit addresses, providing roughly 4.3 billion unique identifiers for network interfaces on the global internet.
IPv4 (Internet Protocol version 4) is the fourth revision of the Internet Protocol and the dominant internet-layer protocol for packet-switched networks since 1983. Defined in RFC 791 (September 1981), it uses a 32-bit address space divided into four octets, conventionally written in dotted-decimal notation (e.g., 192.0.2.1). This design gave a theoretical maximum of 2^32, or about 4.3 billion, unique addresses. Addresses are assigned hierarchically: Internet Assigned Numbers Authority (IANA) allocates large blocks to Regional Internet Registries (RIRs), which assign smaller subnets to ISPs and end users. The original classful addressing system (Classes A, B, C, D, E) has largely been superseded by Classless Inter-Domain Routing (CIDR), which allows arbitrary prefix lengths for more efficient allocation.
Packet delivery in IPv4 is connectionless and best-effort. Each IPv4 header is typically 20 bytes long, containing fields for source and destination addresses, time-to-live (TTL), protocol type, and a checksum. Fragmentation may occur at routers if a packet exceeds the MTU of the next hop. Address resolution between IPv4 and link-layer addresses is handled by the Address Resolution Protocol (ARP) on local networks. Private address ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and network address translation (NAT) were introduced to stretch the limited address pool as the internet grew explosively in the 1990s. The exhaustion of the global IPv4 address pool was formally reached by 2011, with the last RIR block allocated in 2019. This scarcity drove the adoption of IPv6, but IPv4 remains in widespread use due to the installed base and NAT-based carrier-grade deployments.
Key facts
- 32-bit address space yields roughly 4.3 billion unique addresses.
- Defined in RFC 791, published September 1981.
- Uses dotted-decimal notation: four octets from 0.0.0.0 to 255.255.255.255.
- All IANA and RIR pools were exhausted between 2011 and 2019.
- Private address ranges (RFC 1918) and NAT are used to extend usability.
How it works in practice
Related terms
References
More in Networking & Routing
Anycast
Anycast is a network addressing and routing method where a single IP address is assigned to multiple servers, and routers send traffic to the nearest server based on routing protocol metrics.
AS Path
A BGP path attribute that lists the sequence of autonomous system numbers a route has passed through, used for loop detection and path selection.
ASN
A globally unique 16 or 32 bit number assigned to an autonomous system for use in BGP routing between organizations on the Internet.
Autonomous System
An Autonomous System (AS) is a group of IP networks under a single administrative routing policy, identified by a unique ASN (Autonomous System Number) for exterior gateway routing.
BGP
BGP (Border Gateway Protocol) is the path vector routing protocol that networks use to exchange reachability information between autonomous systems on the public internet.
CIDR
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing packets using variable-length subnet masks (e.g., /24) instead of fixed classful boundaries.
Hop
A hop is one passage of a packet through a router or other layer-3 forwarding device as it travels from source to destination across an internetwork.
IPv6
IPv6 is the most recent version of the Internet Protocol, using 128-bit addresses to provide an effectively unlimited number of unique identifiers for networked devices.
Latency
Latency (or round-trip time, RTT) is the time required for a packet to travel from a source to a destination and back, measured in milliseconds, and is a critical metric in network performance.
Looking Glass
A looking glass is a public web-based tool that provides read-only access to a network's BGP routing table, ping, and traceroute diagnostics from that network's perspective.