/27 Subnet Explained: 255.255.255.224

/27 at a glance

What /27 means

A /27 uses 27 bits for the network and leaves 5 bits for hosts. Five bits give 2⁵ = 32 addresses. As in every ordinary subnet, the first is the network address and the last is the broadcast address, so 30 remain for devices. In the mask, the last octet is 11100000, which is 224, hence 255.255.255.224.

Because the "interesting" octet is the last one and the block size is 32, every /27 network starts at a multiple of 32: .0, .32, .64, .96, .128, .160, .192, or .224. These are all the possible /27s inside one /24:

Working out a /27 by hand

Divide the last octet by 32 and drop the remainder, then multiply back. That gives the network address; add 31 for the broadcast.

• 192.168.10.77/27: 77 ÷ 32 = 2.4, so the block number is 2 and the network starts at 2 × 32 = 64. The network is 192.168.10.64/27, the usable range is .65 to .94, and the broadcast is 64 + 31 = .95.
• 10.1.1.200/27: 200 ÷ 32 = 6.25, so the block starts at 6 × 32 = 192. The range is 10.1.1.193 to 10.1.1.222 with broadcast 10.1.1.223.
• 172.16.5.31/27: 31 ÷ 32 is less than 1, so the block starts at 0. The broadcast address is 0 + 31, which is .31. This address looks harmless but it is the broadcast address of 172.16.5.0/27 and cannot be given to a host.

You can confirm any of these with the subnet calculator.

Where a /27 fits

• Small offices and branches with up to about 25 devices.
• Voice VLANs for IP phones on a floor or in a small building.
• Management networks for switches, access points, and out-of-band interfaces.
• DMZs and server groups with a few dozen hosts.
• Cloud subnets for small tiers such as a bastion or an internal load balancer.

When sizing, do not count all 30 addresses as free. The default gateway uses one, and if you run a pair of routers with a first-hop redundancy protocol such as HSRP or VRRP, three addresses are in use before any endpoint connects. A cloud provider may reserve more: AWS and Azure each reserve five addresses per subnet, which leaves 27 in a /27. If you expect more than 20 to 25 devices, move up to a /26 (62 hosts) so you are not renumbering in a year.

Example: mixing /27 and /26 in one /24

Variable-length subnet masks let you cut a single /24 into blocks of different sizes. Here 192.168.10.0/24 is divided so that larger groups get a /26 and smaller ones get a /27, with nothing wasted and a spare block left over:

Allocate the largest blocks first and keep each network aligned to its own block size, which is why the /26 users network starts at .0 and the second /26 starts at .128. A /26 must start on a multiple of 64, and a /27 on a multiple of 32.

Choosing between /28, /27, and /26

Each step in prefix length doubles or halves the size. A /28 gives 14 usable hosts, a /27 gives 30, and a /26 gives 62. Pick the smallest block that still leaves comfortable headroom: with 12 devices today a /28 is tight and a /27 is the safer choice, but with 28 devices even a /27 is almost full and a /26 is wiser. Changing the size later means renumbering every device and updating firewall rules, DHCP scopes, and DNS records, so a little extra space at design time is cheap compared with the cost of moving later.

Common mistakes with /27 networks

• Misaligned networks. 192.168.10.16/27 is not a valid network. The block that contains .16 starts at .0, so the correct network is 192.168.10.0/27.
• Treating .31, .63, .95 and so on as ordinary hosts. They are broadcast addresses of the blocks that end there.
• Sizing to exactly 30. Gateway, redundancy addresses, and growth all eat into the 30.
• Confusing the mask. 255.255.255.224 is /27 and 255.255.255.240 is /28. A single octet value changes the size by half.

Related tools and guides

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