Subnet Calculator

1. Enter an IP Address: Input any valid IPv4 address from the network you're planning. The specific host portion doesn't matter—192.168.1.50 and 192.168.1.1 yield identical network calculations for the same subnet mask.
2. Select a Subnet Mask: Choose from /8 to /32 using CIDR notation. Common choices: /24 for small networks (254 hosts), /16 for medium networks (65,534 hosts), /30 for point-to-point links (2 hosts).
3. Review the Calculations: Examine the network address (first IP), broadcast address (last IP), usable host range, and total host count. The network and broadcast addresses are reserved and cannot be assigned to devices.
4. Apply to Your Infrastructure: Use the network address for routing table entries, the broadcast address for network-wide announcements, and assign devices within the usable host range only.

IPv4 subnetting divides the 32-bit address space using a bitmask. The subnet mask determines which bits represent the network portion (fixed) and which represent the host portion (variable). A /24 network uses 24 network bits and 8 host bits, yielding 2^8 = 256 total addresses, with 254 usable after excluding network and broadcast addresses.

CIDR (Classless Inter-Domain Routing) replaced classful networking in 1993, enabling variable-length subnet masks for efficient IP allocation. The formula for usable hosts is (2^n) - 2, where n equals the number of host bits. For point-to-point links, /31 networks (RFC 3021) permit 2 usable addresses by eliminating the broadcast requirement. Network planning should account for growth—a /24 with 200 devices leaves minimal expansion room compared to a /23 with 510 usable addresses.

• Forgetting Reserved Addresses: The first address (network) and last address (broadcast) are never assignable. A /30 has 4 total addresses but only 2 usable hosts—critical for point-to-point links where every IP counts.
• Overlapping Subnets: When planning multiple subnets, verify ranges don't overlap. 192.168.1.0/24 and 192.168.1.128/25 overlap because the /25 is contained within the /24. Use non-overlapping blocks like 192.168.1.0/25 and 192.168.1.128/25.
• Confusing Subnet Mask Formats: /24 equals 255.255.255.0, not 255.255.255.24. The CIDR number indicates the count of network bits, while dotted-decimal shows which octets are masked. Always verify conversions when configuring network equipment.