Subnet Mask

A subnet mask is an IPv4 bitmask, such as 255.255.255.0, in which the 1 bits mark the network portion of an address and the 0 bits mark the host portion. It is the older, dotted-decimal way of writing a prefix length like /24.

Why it matters

A host applies the mask to its own address and to a destination address. If the network portions match, the destination is on the local link and gets an ARP request; if not, the packet is sent to the default gateway. A wrong mask is a classic cause of a device that reaches some hosts but not others.

Example

Address:  192.168.10.77   = 11000000.10101000.00001010.01001101
Mask:     255.255.255.0   = 11111111.11111111.11111111.00000000
Network:  192.168.10.0

Any address starting 192.168.10 is local. 192.168.11.5 is not, so it goes to the gateway.

Common mistakes

• Entering a mask that does not match the rest of the network, so some hosts look unreachable.
• Using a non-contiguous mask. Valid masks are an unbroken run of 1s followed by 0s.
• Confusing the subnet mask with the default gateway field.

Related

• CIDR
• Subnet
• Default gateway
• ARP

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