Subnet Masks Explained: What Is 255.255.255.0?

What a Subnet Mask Is

A subnet mask is a 32-bit number that, when applied to an IP address using a bitwise AND operation, reveals the network address — the portion of the IP that identifies the network, as opposed to the individual host within that network.

The Binary Reality

IP address:   192.168.1.50   =  11000000.10101000.00000001.00110010
Subnet mask:  255.255.255.0  =  11111111.11111111.11111111.00000000

AND result:   192.168.1.0    =  11000000.10101000.00000001.00000000
                                 (network address)

The 1-bits in the mask identify the network portion. The 0-bits identify the host portion.

Common Subnet Masks

Converting Between CIDR and Subnet Mask

The CIDR prefix (/24) is simply the count of consecutive 1-bits in the subnet mask:

• /24 = 24 ones = 11111111.11111111.11111111.00000000 = 255.255.255.0
• /25 = 25 ones = 11111111.11111111.11111111.10000000 = 255.255.255.128
• /28 = 28 ones = 11111111.11111111.11111111.11110000 = 255.255.255.240

Wildcard Mask

A wildcard mask is the bitwise inverse of the subnet mask. It is used in Cisco ACLs and firewall rules to specify which bits are "don't care":

• Subnet mask: 255.255.255.0 → Wildcard: 0.0.0.255

Why This Matters in Practice

Routers use subnet masks to determine whether a destination IP is on the local network (send directly) or needs to be sent to the default gateway (send to router). Network engineers use subnetting to partition large address spaces into logical segments for security, performance, and manageability.

Use our CIDR / Subnet Calculator to compute all subnet values instantly from any CIDR notation.