IPv4, explained

An IPv4 address is a 32-bit number that identifies a network interface. Humans write it in dotted-decimal notation: four numbers from 0 to 255, each representing 8 bits (an octet). 192.0.2.10 is really 11000000 00000000 00000010 00001010.

32 bits allow about 4.3 billion addresses (2³²). That sounded like plenty in 1981, when RFC 791 defined the protocol. It is not enough for today’s billions of phones, computers and servers — which is why IPv6 exists and why most IPv4 networks now share addresses.

A public address is globally unique and routed on the internet. Your home router or your mobile carrier has one; websites are reached through them.

Private addresses (RFC 1918) can be reused inside any network and are never routed on the public internet:

Devices with private addresses reach the internet through NAT (network address translation): the router rewrites outgoing packets so they carry its public address. That’s why the address this site shows you is usually your router’s, not your laptop’s.

CGNAT and 100.64.0.0/10

Many ISPs, especially mobile networks, add a second layer of NAT — carrier-grade NAT. Your router then gets an address from the shared range 100.64.0.0/10 (RFC 6598), and hundreds of customers share one public IPv4 address. If the WAN address in your router differs from the one shown here, you are probably behind CGNAT.

Classless Inter-Domain Routing (CIDR, RFC 4632) writes a block of addresses as address/prefix-length. The prefix length says how many leading bits are fixed — the network part; the rest is the host part.

• /24 → 8 host bits → 256 addresses (e.g. 192.168.1.0 – 192.168.1.255)
• /16 → 16 host bits → 65,536 addresses
• /32 → a single address

In a traditional subnet the first address is the network address and the last is the broadcast address, so a /24 has 254 usable hosts. Point-to-point links can use a /31 with both addresses usable (RFC 3021).

Before CIDR, addresses were handed out in fixed classes — A (/8), B (/16) and C (/24). The classes wasted space and were abandoned in 1993, but you’ll still hear the terms.

Some blocks are reserved by IANA for specific purposes and never assigned to customers. The table below is generated from the IANA special-purpose address registry that this site uses to classify every lookup.

IANA handed the last free /8 blocks to the five regional internet registries in February 2011. The RIRs then ran out one by one — APNIC in 2011, RIPE NCC in 2019 for regular allocations. Today, new IPv4 space comes from transfers on a secondary market, where a single address sells for tens of dollars.

The industry’s answer has three parts: NAT to share addresses, CGNAT at providers, and IPv6, whose address space makes sharing unnecessary.