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Question
What are the two parts of an IPv4 address and how does the subnet mask determine them?
Answer
An IPv4 address has a network portion and a host portion; the subnet mask marks which bits are which — 1s mark network bits, 0s mark host bits.
* The mask's 1-bits mark the network portion, its 0-bits the host portion. *
IPv4 Address Structure:
| Part | Description |
|---|---|
| Network portion | Identifies the network; same for all devices on that network |
| Host portion | Identifies the specific device on that network |
The subnet mask determines which bits belong to which portion:
- Subnet mask bits set to 1 = Network portion
- Subnet mask bits set to 0 = Host portion
Example:
- IP: 192.168.10.10
- Subnet mask: 255.255.255.0
- Network portion: 192.168.10 (first 24 bits)
- Host portion: .10 (last 8 bits)
Key insight: The subnet mask is essential - without it, you cannot determine which portion is network vs host.
Go deeper:
Subnet (Wikipedia) — the network/host boundary and subnet masks with binary examples.
Subnetting Mastery (PracticalNetworking) — a 7-part series that builds the network/host split from first principles.
Michel Bakni · CC BY-SA 4.0 · Wikimedia Commons
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