An IPv4 address is 32 bits split into a network portion and a host portion. This chapter explains classful history (A/B/C), CIDR notation, and how every dotted-decimal address really lives in binary.
An IPv4 address is 32 bits. Humans write it as four decimal numbers, each 0–255, separated by dots: 192.168.10.42. Each octet is 8 bits, so 192.168.10.42 in binary is 11000000.10101000.00001010.00101010.
The address has two parts: the network portion identifies which network it belongs to, and the host portion identifies the specific device within that network. The boundary between them is set by the subnet mask.
Originally the first octet alone decided the boundary. Class A (1–126) had an /8 mask, Class B (128–191) had /16, Class C (192–223) had /24. This was wasteful — a Class B gave you 65,534 hosts whether you needed them or not — and it has been obsolete since CIDR arrived in 1993.
| Class | First octet | Default mask | Hosts per network |
|---|---|---|---|
| A | 1–126 | /8 | 16,777,214 |
| B | 128–191 | /16 | 65,534 |
| C | 192–223 | /24 | 254 |
| D (multicast) | 224–239 | — | — |
| E (reserved) | 240–255 | — | — |
1 more section in this chapter, plus a 3-question quiz.
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