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IPv4 addressing
Everything in the addressing track (subnetting, NAT, even IPv6’s design) stands on this lesson: what those four numbers actually are, which ranges mean what, and why 4.3 billion addresses ran out.
What is an IPv4 address?
An IPv4 address is a 32-bit number that identifies one interface on a network, written as four octets from 0–255, like 192.168.1.70. A prefix length decides how many leading bits name the network and how many name the host within it: the split every routing decision depends on.
The dotted form is purely for humans. Routers, masks and every calculation in the subnetting lesson operate on the underlying 32 bits.
How does dotted notation map to binary?
Each octet is eight bits, worth 128, 64, 32, 16, 8, 4, 2 and 1 from left to right. Convert one octet at a time: 70 = 64 + 4 + 2, so the octet reads 01000110. Fluency with this table is what makes subnetting feel like arithmetic instead of magic.
bit values 128 64 32 16 8 4 2 1 192 = 1 1 0 0 0 0 0 0 168 = 1 0 1 0 1 0 0 0 1 = 0 0 0 0 0 0 0 1 70 = 0 1 0 0 0 1 1 0 = 64+4+2
Which ranges are private?
Three blocks are reserved for private networks by RFC 1918: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are free to reuse in every home and office, never routed on the public internet, and reach it only through NAT.
| Range | What it is | Where you’ll meet it |
|---|---|---|
10.0.0.0/8 |
Private (RFC 1918) | Enterprises, clouds: 16.7M addresses |
172.16.0.0/12 |
Private (RFC 1918) | Docker defaults, mid-size networks |
192.168.0.0/16 |
Private (RFC 1918) | Practically every home router |
127.0.0.0/8 |
Loopback | 127.0.0.1: this machine talking to itself |
169.254.0.0/16 |
Link-local (APIPA) | Self-assigned: the “DHCP failed” tell |
100.64.0.0/10 |
Carrier-grade NAT | ISP shared space: you, behind your ISP’s NAT |
Who assigns public addresses?
IANA delegates blocks to five regional registries (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC), which allocate to ISPs and large organisations, which assign to customers. The free pool ran dry in the 2010s, so public IPv4 space now trades on a paid transfer market.
That scarcity explains the modern landscape: NAT stretching one public address across a household, carrier-grade NAT stretching one across a neighbourhood, and the long migration to IPv6, where scarcity simply does not exist.
Static or DHCP?
Servers, printers and network gear get static addresses (or DHCP reservations) so they can always be found; everything else leases an address automatically via DHCP. If a device shows 169.254.x.x, it asked for a lease and nobody answered: start troubleshooting at the DHCP server.
Related tool: Subnet Calculator
Ready to split networks? Continue to Subnetting & CIDR with the Subnet Calculator open beside it: do the math on paper, let the tool referee.
Frequently asked questions
How many IPv4 addresses are there?
2³² = 4,294,967,296: about 4.3 billion. Reserved blocks (private, loopback, multicast, link-local) remove hundreds of millions, and with tens of billions of connected devices the pool exhausted years ago. NAT and IPv6 are the two answers to that shortfall.
What exactly is 127.0.0.1?
The loopback address: a virtual interface where the machine talks to itself without touching any network hardware. If a service answers on 127.0.0.1 but not on the machine’s LAN address, the software works and the problem is network or firewall configuration.
Why does my PC show a 169.254.x.x address?
That is APIPA, the link-local range a device assigns itself when its DHCP request goes unanswered. It effectively means “I have no real address”: check the DHCP server, the cable or VLAN, and lease pool exhaustion, in that order.
Can two devices share one IP address?
On the same LAN, an address conflict breaks both devices in confusing, intermittent ways: ARP flips between them. Across NAT, however, thousands of private devices legitimately share one public address; the router’s translation table keeps their conversations separate.