Subnet masks

## What does a mask tell us? An IP address has two parts: the **network part** and the **host part**. The **mask** shows where it splits. ``` IP: 192.168.1.25 Mask: 255.255.255.0 (= /24) └─ network ┘ └host┘ ``` `/24` means the first **24 bits** are the network part. | Prefix | Mask | Number of hosts | |---|---|---| | /24 | 255.255.255.0 | 254 | | /25 | 255.255.255.128 | 126 | | /26 | 255.255.255.192 | 62 | | /27 | 255.255.255.224 | 30 | | /16 | 255.255.0.0 | 65 534 | **Number of hosts = 2^(32 − prefix) − 2**. Why −2? Because: - **Network address** — the very first address (for example `192.168.1.0`) - **Broadcast address** — the very last address (for example `192.168.1.255`), used to send to everyone are never given to computers. ## Are they on the same network? If two addresses have the same **network part**, they are on the same network and can talk directly through a switch. If not, you need a **router**. ``` 192.168.1.10/24 and 192.168.1.200/24 → same network ✅ 192.168.1.10/24 and 192.168.2.10/24 → different network ❌ ``` > 🧮 Problems: **Networks → Level 5–8**