What is a subnet mask?

A subnet mask is a 32-bit number that divides an IP address into network and host portions. For example, 255.255.255.0 is the mask for a /24 network.

IP Subnet Calculator - Free CIDR & Netmask Tool (2025)

Understanding IP Subnetting

Subnetting divides a large network into smaller, more manageable sub-networks. This improves security, reduces congestion, and makes IP address management more efficient.

Key Concepts:
• Network Address: The first IP in the subnet (e.g., 192.168.1.0)
• Broadcast Address: The last IP (e.g., 192.168.1.255)
• Subnet Mask: Defines network vs. host bits (e.g., 255.255.255.0)
• CIDR Notation: Shorthand like /24 instead of 255.255.255.0

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Expert Tip: Plan for Growth

Always allocate more IP addresses than you currently need. If you have 50 devices, use a /25 (126 hosts) or /24 (254 hosts) instead of a /26 (62 hosts). Growth almost always exceeds initial estimates.

Common Mistakes

Using Network/Broadcast IPs: Don't assign the first (network) or last (broadcast) IP to devices—they're reserved.
Forgetting the -2 Rule: Usable hosts = Total - 2, not Total. A /30 gives 4 IPs but only 2 usable.
Mismatching Subnet Masks: All devices on the same subnet must use identical subnet masks.

Reviewed by Robert Park, Network Engineer

Updated November 26, 2025

Frequently Asked Questions

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation is a compact way to specify IP addresses and their routing prefix. For example, 192.168.1.0/24 means the first 24 bits are the network portion.

How do I calculate usable hosts in a subnet?

Usable hosts = Total hosts - 2. Subtract the network address and broadcast address. For a /24 subnet (256 total), you have 254 usable host addresses.

What's the difference between /24 and /16 subnets?

/24 provides 256 addresses (254 usable), while /16 provides 65,536 addresses (65,534 usable). The number after the slash indicates how many bits are used for the network portion.

What is a broadcast address?

The broadcast address is the last IP in a subnet range. Packets sent to this address are delivered to all hosts on that network. For 192.168.1.0/24, the broadcast is 192.168.1.255.