IPv4 is the fourth version of the Internet Protocol, the foundational protocol for routing data packets across networks and the internet. It uses 32-bit addresses—typically written in dotted-decimal notation like 192.168.1.1—giving a theoretical maximum of about 4.3 billion unique addresses.
How it works
IPv4 packets contain a 20-byte header (up to 60 bytes with options) with the source and destination IP address, along with fields for packet length, time-to-live (TTL), and protocol type. Routers forward packets hop-by-hop to their destination using routing tables. IPv4 provides best-effort delivery—packets may be lost, delayed, or arrive out of order, and there is no built-in encryption.
Addressing
IPv4 addresses are divided into public and private ranges. Private addresses—defined in RFC 1918—include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16—are not globally routable and are used within local networks. Modern networks use CIDR (Classless Inter-Domain Routing) to assign variable-length subnets (e.g., /24) rather than the older fixed-class system.
Address exhaustion
The internet has run out of unallocated IPv4 addresses since 2011. Most devices today sit behind NAT (Network Address Translation) or CGNAT, meaning they have private IPs but share a smaller pool of public addresses. This drives ongoing adoption of IPv6, which offers vastly more addresses.
WarningMany modern applications assume one-to-one global addressability that IPv4 no longer provides. Behind NAT, inbound connections and peer-to-peer services require explicit port forwarding or NAT traversal techniques.