What Is an IPv4 Address?
An IPv4 (Internet Protocol version 4) address is a 32-bit numerical label assigned to every device connected to a network that uses the Internet Protocol. Written in dotted-decimal notation, it looks like this: 192.168.1.1. Each of the four numbers (called octets) ranges from 0 to 255, giving a theoretical maximum of about 4.3 billion unique addresses.
The Structure of an IPv4 Address
Every IPv4 address has two logical parts:
- Network portion: Identifies which network the device belongs to.
- Host portion: Identifies the specific device within that network.
A subnet mask (e.g., 255.255.255.0) determines where the network portion ends and the host portion begins. In CIDR notation, the same mask is written as /24, meaning the first 24 bits are the network identifier.
IPv4 Address Classes
Historically, IPv4 addresses were divided into five classes based on the leading bits of the address:
| Class | Range | Default Mask | Typical Use |
|---|---|---|---|
| A | 1.0.0.0 – 126.255.255.255 | /8 | Large organizations, ISPs |
| B | 128.0.0.0 – 191.255.255.255 | /16 | Medium-sized networks |
| C | 192.0.0.0 – 223.255.255.255 | /24 | Small networks |
| D | 224.0.0.0 – 239.255.255.255 | N/A | Multicast |
| E | 240.0.0.0 – 255.255.255.255 | N/A | Reserved/Experimental |
Classful addressing is largely obsolete today, replaced by Classless Inter-Domain Routing (CIDR), which allows far more flexible allocation of address space.
Private vs. Public IPv4 Addresses
Not all IPv4 addresses are reachable on the public internet. Private address ranges, defined in RFC 1918, are reserved for use within local networks:
10.0.0.0 – 10.255.255.255(Class A private)172.16.0.0 – 172.31.255.255(Class B private)192.168.0.0 – 192.168.255.255(Class C private)
Devices using private addresses communicate with the public internet through Network Address Translation (NAT), which maps many private IPs to a single public IP.
Special-Purpose IPv4 Addresses
Several ranges serve special functions:
- 127.0.0.0/8 — Loopback (e.g.,
127.0.0.1refers to the local machine itself) - 169.254.0.0/16 — Link-local / APIPA (auto-assigned when DHCP fails)
- 0.0.0.0 — Unspecified address, used to mean "any available interface"
- 255.255.255.255 — Limited broadcast to the local network
Why IPv4 Addresses Are Running Out
The 32-bit address space provides roughly 4.3 billion addresses — a number that seemed enormous in 1981 when IPv4 was standardized. The explosive growth of the internet, smartphones, IoT devices, and cloud infrastructure has exhausted the available pool. IANA distributed the last blocks of free IPv4 addresses to regional registries in 2011, and most regional registries have since depleted their allocations as well.
This exhaustion is the primary driver behind the global adoption of IPv6, which uses a 128-bit address space and provides a virtually inexhaustible number of unique addresses.
Key Takeaways
- IPv4 addresses are 32-bit, written in dotted-decimal form.
- Subnet masks (or CIDR prefixes) define network vs. host boundaries.
- Private address ranges enable internal networking without consuming public IPs.
- Address exhaustion is real and is accelerating the shift to IPv6.