Subnet Masks in Binary Explained

Subnet masks are 32-bit values where all the 1-bits are contiguous on the left and all the 0-bits are contiguous on the right. This structure divides an IP address into a network portion (1s) and a host portion (0s).

Common subnet masks in binary:

CIDR	Decimal	Binary	Hosts
/8	255.0.0.0	11111111.00000000.00000000.00000000	16.7M
/16	255.255.0.0	11111111.11111111.00000000.00000000	65,534
/24	255.255.255.0	11111111.11111111.11111111.00000000	254
/25	255.255.255.128	11111111.11111111.11111111.10000000	126
/26	255.255.255.192	11111111.11111111.11111111.11000000	62
/27	255.255.255.224	11111111.11111111.11111111.11100000	30
/28	255.255.255.240	11111111.11111111.11111111.11110000	14
/30	255.255.255.252	11111111.11111111.11111111.11111100	2

How to calculate usable hosts:

The number of usable host addresses is 2^n - 2, where n is the number of 0-bits (host bits). We subtract 2 because the all-zeros address (network) and all-ones address (broadcast) are reserved.

Variable-length subnet masking (VLSM):

Modern networks use VLSM to allocate subnets of different sizes efficiently. By understanding binary subnet masks, you can carve a /24 network into smaller subnets: two /25s, four /26s, or a mix. For example, a company might use a /26 (62 hosts) for the main office, a /27 (30 hosts) for a branch, and a /30 (2 hosts) for point-to-point links.

The wildcard mask:

Cisco routers use wildcard masks, which are the bitwise inverse of subnet masks. A /24 subnet mask 255.255.255.0 has a wildcard of 0.0.0.255. In binary, every 1 becomes 0 and vice versa. Wildcard masks indicate which bits are "don't care" in access control lists.