Suggest an editImprove this articleRefine the answer for “What is a subnet mask?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**A subnet mask** is a parameter that determines which part of an IPv4 address belongs to the network and which belongs to the hosts within it. **Key point:** ones in the mask denote the network part, zeros denote the host part.Shown above the full answer for quick recall.Answer (EN)Image**A subnet mask** is a parameter that determines **which part of an IPv4 address belongs to the network and which belongs to the hosts within it**. Short version for an interview: > **A subnet mask is a 32-bit number that separates the network part of an IP address from the host part. Thanks to the mask, a router understands which network this is and which devices are in it.** --- ## What a subnet mask looks like A mask has the same format as an IPv4 address: four numbers from 0 to 255. Examples of common masks: ``` 255.0.0.0 255.255.0.0 255.255.255.0 255.255.255.128 255.255.255.252 ``` But the shorthand CIDR form is used more often: ``` /8 /16 /24 /25 /30 ``` --- ## The meaning of the subnet mask The mask determines: - **which bits belong to the network** → they are set to *1* - **which bits belong to the hosts** → they are set to *0* Example of the **255.255.255.0 (/24)** mask in binary form: ``` 11111111.11111111.11111111.00000000 ``` First 24 bits → network Last 8 bits → hosts --- ## Example of how a subnet mask works Suppose there is an IP: ``` 192.168.10.25 ``` And a mask: ``` 255.255.255.0 (/24) ``` This means: - Network part: **192.168.10** - Host part: **25** Network: ``` 192.168.10.0/24 ``` Broadcast: ``` 192.168.10.255 ``` Host range: ``` 192.168.10.1 - 192.168.10.254 ``` --- ## Why a subnet mask is needed #### **1. To determine subnet boundaries** The router understands: - which IPs belong to the same network, - where to send packets, - where routing is required. #### **2. To divide a network into subnets** For example, a /24 network can be split into two /25s: - 192.168.1.0/25 - 192.168.1.128/25 #### **3. To optimize routing** CIDR and VLSM allow routing tables to be reduced. --- ## An important formula Number of hosts in a network: ``` 2^(number of zero bits in the mask) - 2 ``` Examples: | Mask | Zeros | Hosts | |---|---|---| | /24 | 8 | 254 | | /25 | 7 | 126 | | /26 | 6 | 62 | | /30 | 2 | 2 (for P2P links) | --- ## Common masks and their applications | Mask | CIDR | Where it is used | |---|---|---| | 255.255.255.0 | /24 | Typical office/home subnet | | 255.255.255.128 | /25 | Splitting a network in half | | 255.255.255.252 | /30 | Links between two routers | | 255.0.0.0 | /8 | Huge networks (private range 10.0.0.0) | --- ## Ready answer for an interview > **A subnet mask is a 32-bit number showing which part of an IPv4 address belongs to the network and which belongs to the hosts.** > **In the mask, ones denote the network part, zeros denote the host part.** > **The mask allows you to determine subnet boundaries, route traffic, and divide a network into smaller subnets.** > **Example: for the 255.255.255.0 (/24) mask, the network is the first 24 bits, the hosts are the last 8.** --- ## Cheat sheet (5 seconds) - Subnet mask = separates **Network** from **Host**. - In the mask: **1 = network**, **0 = hosts**. - Notation: 255.255.255.0 = **/24**. - Determines: network, broadcast, host range. - Hosts = 2ⁿ - 2, where n = number of zeros.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.