IPv4

Technical definition

IPv4 is a connectionless, best-effort protocol operating at the network layer (Layer 3) of the OSI model. Each interface using IPv4 is assigned a 32-bit address, conventionally written in dotted-decimal notation as four octets separated by full stops (for example, 198.51.100.7). The IPv4 header carries fields including source and destination address, header length, total length, time-to-live (TTL), a protocol identifier for the next-layer payload, and a header checksum.

IPv4 itself makes no guarantee of delivery, ordering or duplicate suppression. Those properties, where required, are provided by transport-layer protocols such as TCP running above IPv4, or accepted as a trade-off by protocols such as UDP.

Operational relevance

Practitioners encounter IPv4 directly whenever they design subnets, configure routing tables, write firewall rules, allocate DHCP leases or configure Network Address Translation (NAT). Because the IPv4 address field is 32 bits wide, the total address space is finite, and most organisations now combine IPv4 with private addressing ranges behind NAT gateways to conserve public addresses. This scarcity is also the practical reason IPv4 and IPv6 are deployed alongside one another on many networks rather than IPv4 being replaced outright.

Architecture relationship

IPv4 sits between the link layer (for example Ethernet or Wi-Fi) below it and transport-layer protocols such as TCP and UDP above it. On a local network segment, the Address Resolution Protocol (ARP) maps an IPv4 address to a link-layer address so frames can be delivered. Beyond the local segment, routing protocols and routing tables determine the next hop for each IPv4 packet. Internet Control Message Protocol (ICMP) provides diagnostic and error-reporting messages for IPv4 paths, and NAT and DHCP are commonly deployed alongside IPv4 to manage address translation and dynamic assignment.

Example

A host configured with the address 192.0.2.10 on a /24 subnet can reach another host at 192.0.2.20 directly: ARP resolves the destination address to a link-layer address, and Ethernet frames carry the IPv4 packet across the shared segment without involving a router. To reach a host outside that subnet, the same host instead forwards the packet to its configured default gateway, which consults its own routing table to select the next hop toward the destination.

Misunderstanding

• An IPv4 address is often assumed to permanently and uniquely identify one device. In practice, DHCP frequently reassigns addresses, and NAT allows many devices to share a single public address, so an address recorded in a log identifies a network path at a point in time, not a fixed device.
• IPv4 is sometimes assumed to provide reliability or confidentiality. It provides neither; reliable delivery is a property of protocols such as TCP running above IPv4, and confidentiality requires separate controls such as IPsec or TLS.

Related terms

• IPv6
• TCP/IP
• Subnetting
• NAT (Network Address Translation)
• ARP (Address Resolution Protocol)
• DHCP
• Routing table
• CIDR

Further reading

The RFC Editor maintains the RFC Series, the authoritative publication channel for Internet technical specifications, including the formal documents that define IPv4 and its associated protocols. Readers who need to cite a specific RFC number or confirm a version-specific detail of the IPv4 specification should verify the exact document directly against the RFC Series before relying on it in change-control or compliance documentation, since that level of detail was not independently confirmed for this entry.