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Networking & DNSLinux + IT series

Subnet sizes cheat sheet

Six subnet sizes worth knowing by heart. They come up in network design, firewall rules, cloud VPCs and almost every networking interview.

Quick reference

NumberWhatWhy it matters
254/24 255.255.255.0The classic office or home network./24
126/25 255.255.255.128Half a /24. Two departments from one range./25
62/26 255.255.255.192A floor, a branch, or a server VLAN./26
30/27 255.255.255.224A small team, printers, or cameras./27
14/28 255.255.255.240A few servers or a DMZ./28
2/30 255.255.255.252A link between two routers./30

Explained

254 usable hosts, a /24, mask 255.255.255.0. The classic office or home network, like 192.168.1.0/24. 256 addresses minus the network address and the broadcast address.

126 usable hosts, a /25, mask 255.255.255.128. Half of a /24. One /24 split into two networks, for example staff and printers.

62 usable hosts, a /26, mask 255.255.255.192. A good size for one floor, a small branch, or a server VLAN.

30 usable hosts, a /27, mask 255.255.255.224. A small team, a group of printers, or security cameras in their own segment.

14 usable hosts, a /28, mask 255.255.255.240. A handful of servers or a DMZ. Cloud providers often reserve a few extra addresses in every subnet, so check before you size tightly.

2 usable hosts, a /30, mask 255.255.255.252. Traditionally used for the link between two routers. A /31 is also allowed for point-to-point links and saves two addresses.

The pattern that makes it easy: every extra bit in the prefix halves the number of addresses, and you subtract two for the network and broadcast addresses.

usable hosts = 2^(32 - prefix) - 2.

Size subnets for growth. Renumbering a full network later is far harder than leaving room now.

In one line

Each bit halves it. Minus two. Network address and broadcast are not usable.

The formula: usable = 2^(32 - prefix) - 2

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