/25 Subnet: Mask 255.255.255.128

How many /25 networks fit

2 × /25 = one /24. A /25 holds 2 × /26 or 4 × /27.

Worked example: 10.10.5.200/25

A /24 was split in two: wired desks on the lower half, Wi-Fi on the upper half. A phone has 10.10.5.200. Which half is it in, and what should its gateway be?

1. The mask is 255.255.255.128. The fourth octet (128) is the "interesting" one — the first that is not 255.
2. Block size = 256 − 128 = 128. Networks start at multiples of 128 in the fourth octet: 0, 128
3. The fourth octet of 10.10.5.200 is 200. 200 ÷ 128 = 1 remainder 72, so the block starts at 1 × 128 = 128.
4. Network = 10.10.5.128. Broadcast = 128 + 128 − 1 = 255, giving 10.10.5.255.
5. Usable hosts: 10.10.5.129 to 10.10.5.254, which is 2⁷ − 2 = 126 addresses.

Address  00001010.00001010.00000101.1|1001000
Mask     11111111.11111111.11111111.1|0000000
Network  00001010.00001010.00000101.1|0000000

Where /25 is used in real networks

• Splitting one /24 between two VLANs

  If you only have one /24 left, two /25s give you two VLANs of 126 hosts each — for example staff and guests, or wired and wireless.

• Public IP blocks

  Hosting providers sometimes hand out a /25 of public space to a customer rack: one gateway, 125 addresses for servers.

Common /25 mistakes

• Using .1 as the gateway for the upper /25. The upper half starts at .128, so its first usable address is .129. A gateway of .1 is in the other subnet and hosts can't reach it.
• Forgetting that .127 and .128 are now reserved. In a /24 they were normal hosts; after the split, .127 is the broadcast of the lower half and .128 is the network address of the upper half.

/25 FAQ

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