IPv4 Addressing and Subnetting
IPv4 addressing schemes
What you will learn
- Calculate usable host count from the CIDR mask length.
- Distinguish private from public addresses, and recall the three private ranges.
- Explain why the network and broadcast addresses are subtracted from the total.
- Determine the network and broadcast address for a given subnet.
An IPv4 address is always 32 bits, and the mask is what splits those bits into a network portion and a host portion. Every calculation in this lesson rests on a single formula: 2 raised to the number of host bits, minus 2 (for the network and broadcast addresses).
Calculating hosts from a /27 mask — step by step
1. Subtract the mask length from 32
32 − 27 = 5 host bits.
2. Raise 2 to the number of host bits
2^5 = 32 total addresses.
3. Subtract 2 (network and broadcast)
32 − 2 = 30 usable hosts.
The three private ranges (RFC 1918)
| 10.0.0.0/8 | The largest private range — common in large enterprises. |
|---|---|
| 172.16.0.0/12 | A medium range — less common than the 10.x block. |
| 192.168.0.0/16 | The most familiar range in home and small-office networks. |
Memory aid
Don't memorize full mask tables — just remember that each extra network bit halves the available hosts: /24 gives 254 hosts, /25 gives 126, /26 gives 62, /27 gives 30 — each step is roughly half the previous one.
What matters on the exam
- Formula: 2^(host bits) − 2 = usable hosts.
- Network address = first address in the range; broadcast address = last one — neither is assignable to a device.
- 192.168.x.x, 10.x.x.x, and 172.16–31.x.x are the only private ones — any other address is assumed public unless stated otherwise.
Quick check
Answer in your head first, then reveal.
How many usable hosts in a /26 mask?Reveal the answer
62 (2^6 − 2).
What's the most common private range in home networks?Reveal the answer
192.168.0.0/16.
Why is 2 subtracted from the total address count?Reveal the answer
To exclude the network and broadcast addresses — neither assignable to a device.
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Your next step
Read the lesson, then mark it complete
Sources
Used to verify the facts. The writing is original to Passuit.