IPv4

Address Structure

An IPv4 address is a 32-bit unsigned integer, conventionally written in dotted-decimal notation as four octets separated by periods (e.g., 203.0.113.45). Each octet ranges from 0 to 255. The address space contains 2^32 = 4,294,967,296 possible addresses, though significant portions are reserved: Private IP AddressAn IP address from reserved ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used within local networks. Private addresses are not routable on the public internet and require NAT for external communication. ranges, Loopback AddressA reserved IP address (127.0.0.1 in IPv4, ::1 in IPv6) that routes traffic back to the local device without reaching the network. Used for testing and inter-process communication on the same machine. (127.0.0.0/8), link-local (169.254.0.0/16), multicast (224.0.0.0/4), and documentation blocks.

The CIDRClassless Inter-Domain Routing. A method of allocating IP addresses using variable-length subnet masking (e.g., 10.0.0.0/8) instead of fixed classful boundaries, enabling more efficient use of the IPv4 address space. prefix (or equivalent Subnet MaskA 32-bit number (e.g., 255.255.255.0) that divides an IP address into network and host portions. It determines which part of the address identifies the network and which part identifies individual devices.) divides each address into a network portion and a host portion. Routers forward packets based on the network portion; the host portion identifies the specific device within that network.

IPv4 Exhaustion

IANA exhausted its central IPv4 pool in February 2011. Regional registries (ARIN, RIPE NCC, APNIC, etc.) followed in subsequent years. New allocations now come from reclaimed or transferred blocks, with secondary-market prices for address space reaching several dollars per address.

Two mechanisms have extended IPv4's life: NATNetwork Address Translation. A method of remapping private IP addresses to a single public IP address (and vice versa) at a router, allowing multiple devices to share one public IP. A key technique for mitigating IPv4 address exhaustion., which allows thousands of devices to share a single Public IP AddressA globally unique IP address assigned by an ISP that is routable on the public internet. Every device directly accessible from the internet must have a public IP address., and CIDRClassless Inter-Domain Routing. A method of allocating IP addresses using variable-length subnet masking (e.g., 10.0.0.0/8) instead of fixed classful boundaries, enabling more efficient use of the IPv4 address space. aggregation, which slowed BGP table growth. Neither solves the fundamental scarcity — they defer migration to IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion..

Coexistence with IPv6

Most modern networks run a dual-stack configuration, supporting both IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. and IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. simultaneously. Transition mechanisms like 6to4, Teredo, and NAT64/DNS64 allow IPv6-only hosts to reach IPv4 destinations and vice versa. Content delivery networks and major platforms are fully dual-stacked, serving whichever protocol the client prefers.

Use IP Lookup to check geolocation and ASN information for any IPv4 address. Subnet Calculator helps with prefix arithmetic.