# /25 Subnet Mask (255.255.255.128) – Hosts, Range & FAQ > The /25 subnet mask is 255.255.255.128. A /25 block holds 128 addresses with 126 usable hosts — ranges, wildcard, binary math and FAQs. - Page URL: https://itinsighthub.com/subnet/25/ - Markdown variant of this page: append `?format=md` to any URL on this site or send `Accept: text/markdown`. - Full site index for AI assistants: https://itinsighthub.com/llms.txt - Full site export: https://itinsighthub.com/llms-full.txt # /25 Subnet Mask ReferenceIPv4 Slice a /24 in half and you get two /25s, each with mask 255.255.255.128 and 126 usable hosts. It is the first stop when one LAN gets crowded and you want production up front and guests in the back without changing upstream gear. The /25 subnet mask is 255.255.255.128, equivalent to 0.0.0.127 as a wildcard. A /25 block contains 128 IP addresses (126 usable hosts). ## /25 facts at a glance - **Subnet mask:** 255.255.255.128 - **Wildcard mask:** 0.0.0.127 - **Binary mask:** 11111111.11111111.11111111.10000000 - **Total addresses:** 128 - **Usable hosts:** 126 (excludes network & broadcast) - **Block size:** 128 addresses (27) - **Typical role:** Subnets, VLANs and WAN links ## Example /25 networks Three consecutive /25 blocks, starting at the /25 boundary that contains 198.51.100.0: | Network | Usable range | Usable hosts | | --- | --- | --- | | 198.51.100.0/25 | 198.51.100.1 – 198.51.100.126 | 126 | | 198.51.100.128/25 | 198.51.100.129 – 198.51.100.254 | 126 | | 198.51.101.0/25 | 198.51.101.1 – 198.51.101.126 | 126 | ## How to calculate a /25 mask A /25 mask sets the first 25 bits of a 32-bit number to 1 and leaves the remaining 7 bits as 0. In binary that reads `11111111.11111111.11111111.10000000`, which converts to dotted decimal `255.255.255.128`. Each block covers a contiguous run of 27 = 128 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving `0.0.0.127`. ## Where /25 is used - Splitting a crowded 192.168.1.0/24 into two DHCP scopes on the same router. - Floor-by-floor separation of workstations from printers and IoT devices. - Small office plans that need two security zones from a single inherited block. The upper half starts at .128: 192.168.1.0/25 serves .1 through .126, while 192.168.1.128/25 serves .129 through .254. Each half keeps its own broadcast (.127 and .255). Ancient gear sometimes mishandled masks that broke octet boundaries; anything built this century handles a /25 without complaint. ## Frequently asked questions **How many usable hosts are in a /25?** 126 of 128 total addresses, after removing the network and broadcast slots. **What is the subnet mask for a /25?** 255.255.255.128, wildcard 0.0.0.127: seven host bits, half of a classic /24. **How do I split a /24 into two /25s?** Declare x.x.x.0/25 for the lower range and x.x.x.128/25 for the upper. Together they reassemble the original /24 exactly. **What is the broadcast address of the lower /25?** x.x.x.127, since the low bit of the fourth octet is the first fixed bit in the mask. ## Open this block in a calculator [Open the /25 block in the subnet calculator](https://itinsighthub.com/subnet-calculator/?q=198.51.100.0%2F25) [Get the /25 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F25) ### Related prefixes [/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/30](https://itinsighthub.com/subnet/30/) ← /24 subnet mask /26 subnet mask → --- © 2026 ITInsightHub · [About](https://itinsighthub.com/about/) · [Contact](https://itinsighthub.com/contact/) · [Privacy](https://itinsighthub.com/privacy/)