IPv4 Addresses Explained
An IPv4 address is a 32-bit number that identifies a network interface. We write it as four 8-bit numbers (octets) separated by dots: 192.168.1.42. Each octet is 0–255. That gives us about 4.3 billion possible addresses — and we ran out years ago.
What an IP address actually is
192 . 168 . 1 . 42
11000000 . 10101000. 00000001. 00101010
= 11000000101010000000000100101010 (32 bits)
= 3232235818 (one big number)The dots are just for human readability.
Network portion vs host portion
An IP address is split into two parts:
- Network portion — identifies the network
- Host portion — identifies a specific machine on that network
The split is determined by the subnet mask (or CIDR prefix).
IP: 192.168.1.42
Subnet mask: 255.255.255.0 (or /24 in CIDR)
Network: 192.168.1.0
Host: .42 (within that network)Subnetting deserves its own deep dive. Coming up next on the roadmap.
The (now legacy) classful addressing
Originally, IP addresses were grouped by “class”:
| Class | Range | Default mask | Hosts |
|---|---|---|---|
| A | 1–127.x.x.x | /8 | ~16 million |
| B | 128–191.x.x.x | /16 | ~65 thousand |
| C | 192–223.x.x.x | /24 | 254 |
| D | 224–239.x.x.x | multicast | — |
| E | 240–255.x.x.x | reserved | — |
Classful addressing is obsolete. Modern networks use CIDR — variable-length subnet masks. Mention classes only when an old documentation forces you to.
Special addresses to know
0.0.0.0— “this network” / unspecified. Used to bind a server to all interfaces.127.0.0.1— loopback. Always points back to your own machine.255.255.255.255— broadcast to all hosts on this network.169.254.0.0/16— link-local (auto-config when DHCP fails).
Private address ranges (RFC 1918)
Reserved for use inside private networks — never route over the internet:
10.0.0.0/8172.16.0.0/12192.168.0.0/16
This is what NAT uses to make many devices share one public IP.
Why we ran out
4.3 billion addresses sounds like a lot. The internet has billions of phones, IoT devices, servers, cloud VMs. We’ve been out of free IPv4 since 2011. The two solutions: NAT (pack many devices behind one public IP) and IPv6 (much bigger address space).
Useful commands
# See your IPs
ip addr show # Linux
ifconfig # macOS / older Linux
ipconfig # Windows
# Find your public IP
curl ifconfig.me
curl https://api.ipify.orgWhat to learn next
Subnetting and CIDR — the most useful network skill you can have. Up next.