Networking Fundamentals · lesson 5 of 17 · 20 min
Subnet masks and CIDR notation
A subnet mask is a 32-bit number that separates the network portion of an IP address from the host portion. Written in dotted decimal (
255.255.255.0
) or CIDR prefix notation (
/24
).
- How subnet masks work
- A subnet mask of
255.255.255.0
(/24) means the first 24 bits are the network, the last 8 bits are the host. For
192.168.1.100/24
: network =
192.168.1.0
, host =
.100
, broadcast =
192.168.1.255
.
Calculating usable hosts
Usable hosts = 2^(host bits) − 2. The −2 accounts for the network address (all host bits 0) and broadcast address (all host bits 1).
/24 → 8 host bits → 2^8 − 2 =
254 usable hosts
/25 → 7 host bits → 2^7 − 2 =
126 usable hosts
/30 → 2 host bits → 2^2 − 2 =
2 usable hosts
(point-to-point links)
Subnet mask quiz
How many usable host addresses does a /26 subnet provide?
- 62
- 64
- 30
- 126
Answer: 62
/26 leaves 6 host bits. 2^6 = 64 total addresses. Subtract 2 (network + broadcast) = 62 usable hosts.
What is the network address for 172.16.5.200/24?
- 172.16.5.0
- 172.16.0.0
- 172.16.5.200
- 172.16.5.255
Answer: 172.16.5.200
Wait — /24 means the first 24 bits are the network. 172.16.5 is the network portion, so the network address is 172.16.5.0.
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