Subnetting and CIDR Notation: A Practical Refresher
If you’ve ever stared at 192.168.1.0/24 and blanked on what the /24 actually means, this is for you.
What CIDR notation is really saying
CIDR (Classless Inter-Domain Routing) notation is just a compact way of writing a subnet mask. The number after the slash tells you how many bits, counting from the left, are fixed as the network portion of the address. Everything after that is up for grabs as host addresses.
/24→255.255.255.0→ 256 addresses, 254 usable hosts/25→255.255.255.128→ 128 addresses, 126 usable hosts/30→255.255.255.252→ 4 addresses, 2 usable hosts (common for point-to-point links)
A quick mental shortcut
Every time you add one bit to the prefix, you cut the address space in half. Starting from /24 (256 addresses), each additional bit halves it: /25 = 128, /26 = 64, /27 = 32, and so on. Working backwards from a /32 (a single host) doubles it each time.
Worked example
Say you’re handed 10.0.0.0/22 and need to split it into four equal subnets for four office floors.
A /22 gives you 1024 addresses. Splitting into four means moving to a /24 for each subnet:
10.0.0.0/2410.0.1.0/2410.0.2.0/2410.0.3.0/24
Each floor gets 254 usable hosts — plenty for a typical office segment, with room to grow.
Why this still matters
Even with DHCP and cloud VPCs doing a lot of the heavy lifting, understanding subnetting is what lets you reason about routing tables, firewall rules, and “why can’t this host reach that one” without guessing.