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Networking Fundamentals · Foundational

Subnet Mask

A 32-bit value with contiguous 1 bits marking the network portion of an IPv4 address and 0 bits marking the host portion.

Definition

A host performs a bitwise AND of its address and mask to find its network address. Masks are written in dotted decimal (255.255.255.0) or as a prefix length (/24). A wrong mask causes a host to misjudge which destinations are local, breaking connectivity to some hosts.

Example

255.255.255.192 equals /26.

Exam tip

Know the conversions cold: /25=128, /26=192, /27=224, /28=240, /29=248, /30=252 in the last octet.

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