What Is IPv4?
IPv4 (Internet Protocol version 4) is the fourth version of the Internet Protocol and the
addressing system that has carried most internet traffic since the early 1980s. An IPv4
address is a 32-bit number, normally written as four decimal values from 0 to 255 separated
by dots, for example 192.0.2.10. Every device that speaks IPv4 needs one so that
routers can deliver packets to the right place.
How IPv4 works
IPv4 is a connectionless, best-effort protocol. It splits data into packets, stamps each one with a source and destination IPv4 address in the packet header, and hands it to the network. Routers along the path read the destination address, consult their routing tables and forward the packet hop by hop until it reaches the target host. IPv4 itself does not guarantee delivery or order; higher-layer protocols such as TCP add reliability on top.
Every IPv4 address is logically divided into a network portion and a
host portion. The network portion identifies which network a device
belongs to; the host portion identifies the specific device within that network. The
boundary between the two is set by a subnet mask or, more compactly, by a CIDR prefix
length such as /24. This split is what lets the internet route efficiently:
routers only need to know how to reach a network, not every individual host.
IPv4 format and examples
An IPv4 address is 32 bits, grouped into four 8-bit octets. Each octet is written
in decimal (0-255) and the four are joined with dots. The address
203.0.113.42 is, in binary,
11001011.00000000.01110001.00101010. Because each octet holds 8 bits, the
highest value any octet can reach is 255.
- Loopback:
127.0.0.1- always refers to the local machine itself. - Private (RFC 1918):
10.0.0.0/8,172.16.0.0/12,192.168.0.0/16- reusable inside private networks, not routed on the public internet. - Link-local (APIPA):
169.254.0.0/16- auto-assigned when no DHCP server answers. - Documentation (RFC 5737):
192.0.2.0/24,198.51.100.0/24,203.0.113.0/24- reserved for examples like the ones on this page. - CIDR example:
192.168.1.0/24describes 256 addresses, of which 254 are usable for hosts (the first is the network address and the last is the broadcast address).
Why IPv4 is running out
A 32-bit address space tops out at 232 = 4,294,967,296 addresses. That seemed limitless when IPv4 was designed, but a world of phones, laptops, servers, cloud instances and IoT sensors exhausted it. IANA handed its last free blocks to the Regional Internet Registries in February 2011, and most registries have since reached depletion. Two techniques stretched the lifespan of IPv4: NAT (Network Address Translation), which lets many private devices share one public address, and CIDR, which replaced rigid address classes with flexible variable-length prefixes. The long-term fix is IPv6, with its vastly larger 128-bit address space.
IPv4 vs IPv6 at a glance
IPv6 is the successor protocol, built to solve IPv4 address exhaustion. The headline
difference is size: IPv4 uses 32-bit addresses (about 4.29 billion total) written in
dotted decimal, while IPv6 uses 128-bit addresses (about 3.4×1038 total)
written in colon-separated hexadecimal such as 2001:db8::1. The two protocols
are not wire-compatible, so they cannot talk directly and most networks run both side by
side. For a full breakdown of headers, NAT, security and adoption, see the
IPv4 vs IPv6 guide.
Related tools and guides
- IPv4 vs IPv6 - the complete side-by-side comparison.
- IPv6 (glossary) - the definition of the successor protocol.
- IP Lookup - geolocation and network details for any IPv4 or IPv6 address.
- IP Validation - check whether a string is a valid IPv4 or IPv6 address.
- IP Subnet Calculator - work out subnet ranges, masks and host counts.
- IPv4 to IPv6 Converter - map an IPv4 address into IPv6 notation.
Frequently asked questions
What is an IPv4 address in simple terms?
An IPv4 address is a 32-bit number that uniquely identifies a device on an IP network. It is written as four decimal numbers from 0 to 255 separated by dots, such as 192.168.1.10. Routers use it to deliver packets to the correct destination, much like a postal address routes a letter.
How many IPv4 addresses are there?
Because an IPv4 address is 32 bits long, there are 2 to the power 32, or 4,294,967,296, possible addresses (about 4.29 billion). A large share of those are reserved for private, multicast and other special uses, so the pool of globally routable public addresses is smaller, which is why IPv4 has effectively run out.
What is the difference between a public and a private IPv4 address?
A public IPv4 address is globally unique and reachable across the internet. A private address comes from the reserved ranges 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 (RFC 1918) and is only valid inside a local network. Private addresses reach the internet through NAT, which shares one public address among many internal devices.
What does the slash number like /24 mean in an IPv4 address?
The slash is CIDR notation and gives the prefix length: the number of leading bits that identify the network. In 192.168.1.0/24 the first 24 bits are the network and the remaining 8 bits address hosts, giving 256 addresses (254 usable). A smaller number such as /16 means a larger network with more hosts.
Is IPv4 being replaced by IPv6?
IPv6 is the long-term successor because IPv4 ran out of free addresses, but IPv4 is not switching off. The internet runs both at once in a dual-stack model, and IPv4 will stay essential for years because so much hardware and software still depends on it.
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