# How to Calculate a Subnet Mask (Step by Step) > Learn how to calculate a subnet mask step by step from any CIDR prefix, with a fully worked example, a quick reference table and common mistakes to avoid. - Page URL: https://itinsighthub.com/guides/how-to-calculate-a-subnet-mask/ - 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 # How to Calculate a Subnet Mask A repeatable five-step method to turn any CIDR prefix into its mask, network address, broadcast address and host range — worked end to end with 192.168.1.130/26. Published August 22, 2026 The short version: mask = first p bits set · host bits = 32 − p · block size = 2(32−p) · network = IP AND mask · broadcast = network + block size − 1. Worked example: for 192.168.1.130/26 the mask is 255.255.255.192, the network is 192.168.1.128, the broadcast is 192.168.1.191, and there are 62 usable host addresses. ## The five steps 1. **Write the prefix as a binary mask.** A /26 sets the first 26 bits to 1 and the rest to 0: `11111111.11111111.11111111.11000000`, which in dotted decimal is `255.255.255.192`. 2. **Count the host bits.** Host bits = 32 − prefix = 32 − 26 = **6 bits**. These are the zero positions in the mask. 3. **Compute the block size.** Block size = 2host bits = 26 = **64 addresses**. Every /26 covers a contiguous run of 64 addresses. 4. **Find the network address.** Bitwise-AND the IP with the mask (see the octet math below). 192.168.1.130 AND 255.255.255.192 = `192.168.1.128`. 5. **Broadcast and hosts.** Broadcast = network + block size − 1 = 192.168.1.128 + 64 − 1 = `192.168.1.191`. Everything strictly between is a usable host address. ### The AND operation, octet by octet Only the fourth octet differs between the IP and the mask here, so that is the only octet where the AND changes anything: *Bitwise AND of IP and mask in the last octet* | | Decimal | Binary | | --- | --- | --- | | IP (last octet) | 130 | 10000010 | | Mask (last octet) | 192 | 11000000 | | AND result | 128 | 10000000 | Reading down each column: wherever the mask has a 1, the IP bit passes through; wherever it has a 0, the result is forced to 0. That is how the host portion is erased and the network boundary appears. The procedure is the one originally specified for subnetting in [RFC 950](https://datatracker.ietf.org/doc/html/rfc950), and it has not changed since. ## Worked example: 192.168.1.130/26 Applying the five steps above: - **Subnet mask:** 255.255.255.192 - **Wildcard mask:** 0.0.0.63 - **Network address:** 192.168.1.128 - **Broadcast address:** 192.168.1.191 - **First usable host:** 192.168.1.129 - **Last usable host:** 192.168.1.190 - **Total addresses:** 64 - **Usable hosts:** 62 ## Quick reference: common prefixes *Frequently used prefix lengths with masks and host counts* | Prefix | Subnet mask | Total addresses | Usable hosts | Last host octet* | | --- | --- | --- | --- | --- | | /24 | 255.255.255.0 | 256 | 254 | .254 | | /25 | 255.255.255.128 | 128 | 126 | .126 | | /26 | 255.255.255.192 | 64 | 62 | .62 | | /27 | 255.255.255.224 | 32 | 30 | .30 | | /28 | 255.255.255.240 | 16 | 14 | .14 | | /30 | 255.255.255.252 | 4 | 2 | .2 | *Shown for a network whose fourth octet is 0 (e.g. 192.168.1.0/24). The full table for every prefix lives on the [subnet cheat sheet](https://itinsighthub.com/subnet-cheat-sheet/). ## Common mistakes - **Off-by-one on usable hosts.** A /26 holds 64 addresses and 62 usable hosts. People subtract 2 twice (getting 60) or forget to subtract at all (counting the network and broadcast as hosts). Subtract exactly once: `total − 2`. - **Forgetting that network and broadcast are reserved.** If a VLAN needs exactly 62 devices, a /26 leaves zero headroom. Size up a prefix, or use the [VLSM calculator](https://itinsighthub.com/vlsm-calculator/) to right-size multiple subnets at once. - **Assuming a /31 has zero usable hosts.** Under the traditional rule, 21 − 2 = 0. But RFC 3021 explicitly defines two-address /31 subnets for point-to-point links, where both addresses are assigned — one per router interface (see [RFC 3021](https://datatracker.ietf.org/doc/html/rfc3021)). The same reasoning applies to /32 host routes, which carry exactly one address. - **Picking an unaligned network address.** Blocks start only at multiples of their own size. The network for 192.168.1.130/26 is not 192.168.1.130 itself; /26 blocks in the last octet begin at .0, .64, .128 and .192, so the answer is 192.168.1.128. ## Frequently asked questions **Why does /24 map to 255.255.255.0?** A /24 sets the first 24 of the 32 mask bits to 1 and the remaining 8 to 0. Written out per octet that is 11111111.11111111.11111111.00000000, and each octet read as a decimal number gives 255.255.255.0. **What does the subnet mask 255.255.255.255 mean?** All 32 bits are set, so the block holds exactly one address — a host route. Routing tables and ACLs use it to match a single IP rather than a range. The opposite extreme is 0.0.0.0/0, which matches every destination. **How many usable hosts does a /26 have?** A /26 has 26 network bits and 6 host bits, so it spans 64 addresses; reserving the first (network) and last (broadcast) leaves 62 usable hosts. Each /26 begins at a multiple of 64 in the fourth octet: .0, .64, .128 or .192. **What is the difference between a subnet mask and a wildcard mask?** The wildcard mask is the bitwise inverse of the subnet mask: where the mask has 1s the wildcard has 0s. A /26 is 255.255.255.192 as a subnet mask and 0.0.0.63 as a wildcard. Wildcards are used in Cisco-style ACLs, where a 0 bit means 'must match' and a 1 bit means 'don't care'. **Is the subnet mask sent with every packet?** No. The mask is local configuration only — it never leaves the device. Each host uses it internally to decide whether a destination address is on-link (deliver directly) or off-link (hand the packet to the default gateway). ## Check your work Hand calculations are great practice — verify them before they reach production: 🧩 ### IPv4 Subnet Calculator Full breakdown of any CIDR block: network, broadcast, host range, wildcard mask and binary view. 📋 ### Subnet Cheat Sheet Every prefix from /0 to /32 with mask, wildcard and host counts on one printable page. Want the theory behind slash notation first? Start with [CIDR notation explained](https://itinsighthub.com/guides/cidr-notation-explained/), or see the deep dive on the [/26 subnet mask](https://itinsighthub.com/subnet/26/). --- © 2026 ITInsightHub · [About](https://itinsighthub.com/about/) · [Contact](https://itinsighthub.com/contact/) · [Privacy](https://itinsighthub.com/privacy/)