A netmask is a 32-bit value paired with an IPv4 address. Each 1 marks a network bit, while each 0 marks a host bit. The same boundary can be written as a prefix length: /24 means 24 network bits and 8 host bits. Netmask and subnet mask are interchangeable terms. section 7.3.1 section 7.4
Common prefix lengths and their dotted-decimal netmasks include:
- /8 = 255.0.0.0; /16 = 255.255.0.0; /24 = 255.255.255.0. section 7.3.1
- A netmask normally consists of consecutive 1s followed by consecutive 0s; for example, /26 is 255.255.255.192. section 11.2.1
- A longer network portion leaves fewer host bits, while a shorter network portion leaves more host bits. section 7.3.4
Using a netmask on a router interface
In Cisco IOS interface configuration mode, enter `ip address 192.168.1.1 255.255.255.0` to assign 192.168.1.1 with a /24 netmask, then use `no shutdown` to enable the interface. Verify the result with `show ip interface brief`; a configured and connected interface appears with its address and an up/up status. section 7.3.2
The netmask lets a host decide whether a destination is local. With 192.168.1.11 and 255.255.255.0, the local range is 192.168.1.0 through 192.168.1.255. A destination outside that range is sent to the default gateway rather than directly to the destination host. section 10.1.1
For subnetting questions, identify the network and host portions, set the host bits to zero to find the network address, and convert the result back to dotted decimal. For 172.16.20.27/28, this produces 172.16.20.16. section 11.2.1
