Reference
Common Subnet Sizes (CIDR Quick Reference)
| Prefix | Subnet mask | Total IPs | Usable hosts | Common use |
|---|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | Class A — large ISP / enterprise |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Class B — campus / large office |
| /24 | 255.255.255.0 | 256 | 254 | Class C — standard office LAN |
| /25 | 255.255.255.128 | 128 | 126 | Split /24 into two halves |
| /26 | 255.255.255.192 | 64 | 62 | Small office / VLAN segment |
| /27 | 255.255.255.224 | 32 | 30 | Small team network |
| /28 | 255.255.255.240 | 16 | 14 | Small cloud subnet |
| /29 | 255.255.255.248 | 8 | 6 | Very small segment |
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point link |
| /32 | 255.255.255.255 | 1 | 0 | Single host / loopback route |
Reference
Private IP Address Ranges (RFC 1918)
10.0.0.0/8
10.0.0.0 – 10.255.255.255
16.7 million addresses
Large enterprise networks, data centers
172.16.0.0/12
172.16.0.0 – 172.31.255.255
1.05 million addresses
Medium networks, Docker default bridge
192.168.0.0/16
192.168.0.0 – 192.168.255.255
65,536 addresses
Home routers, small office LANs
Private IP ranges are not routable on the public internet. Routers use NAT (Network Address Translation) to map private IPs to a public IP for internet access. IPv6 uses fc00::/7 for its private (ULA) address range.
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Tool guide
How to read CIDR and subnet calculator results
Use the CIDR calculator to turn an IP address and prefix into network address, broadcast address, subnet mask, wildcard mask, usable host range, and total address count.
Recommended workflow
- Enter the IPv4 address with CIDR prefix, such as 192.168.1.0/24 or 10.10.10.0/28.
- Check the network address and broadcast address before assigning devices, firewall rules, or static routes.
- Use the first and last usable host values to avoid accidentally using reserved network or broadcast addresses.
Before relying on the result
- Verify whether your network uses /31 or /32 behavior before applying host counts to point-to-point or single-host routes.
- Confirm subnet changes against router, DHCP, VLAN, and firewall configuration before deploying to production.
- Document the CIDR prefix and subnet mask together so other administrators can read the address plan clearly.
Help & answers
Frequently Asked Questions
What is a CIDR calculator?
A CIDR (Classless Inter-Domain Routing) calculator takes an IP address with a prefix length (e.g., 192.168.1.0/24) and computes the network address, broadcast address, subnet mask, first/last usable host, wildcard mask, and total number of hosts. It is an essential tool for network engineers and system administrators.
What does the /24 mean in an IP address?
The /24 is the prefix length, indicating how many bits of the 32-bit IP address are used for the network portion. /24 means 24 bits for the network and 8 bits for hosts, giving 256 total addresses (254 usable hosts). Common prefixes: /8 = Class A (16M hosts), /16 = Class B (65K hosts), /24 = Class C (254 hosts).
What is a subnet mask?
A subnet mask is a 32-bit number that separates the network and host portions of an IP address. Written in dotted decimal, /24 = 255.255.255.0 and /16 = 255.255.0.0. The network bits are all 1s and host bits are all 0s. CIDR notation (/24) and dotted decimal (255.255.255.0) are two ways to express the same thing.
How many usable hosts does a /24 subnet have?
A /24 subnet has 256 total addresses (2^8), but only 254 are usable for hosts. The first address is the network address (192.168.1.0) and the last is the broadcast address (192.168.1.255). Always subtract 2 from the total for usable hosts.
What is a broadcast address?
The broadcast address is the last address in a subnet, used to send a packet to all devices on that subnet simultaneously. Routers do not forward broadcasts between subnets. In a /24 network like 192.168.1.0/24, the broadcast address is 192.168.1.255.
What is a wildcard mask?
A wildcard mask is the inverse of the subnet mask. Where the subnet mask has 1s, the wildcard has 0s, and vice versa. Wildcard masks are used in ACLs (access control lists) and OSPF configuration in Cisco and other routers. For /24, the wildcard mask is 0.0.0.255.
What is the difference between /30 and /31 subnets?
A /30 subnet has 4 addresses (2 usable hosts) and is traditionally used for point-to-point links. A /31 subnet has only 2 addresses and no broadcast, designed specifically for point-to-point links per RFC 3021 — both addresses are usable. /31 is more efficient for router-to-router connections.
How do I split a /24 into smaller subnets?
Each additional bit you add to the prefix halves the subnet size. A /24 (254 hosts) splits into: two /25 (126 hosts each), four /26 (62 hosts each), eight /27 (30 hosts each), sixteen /28 (14 hosts each). This is called subnetting and is used to organize network segments by department, floor, or function.
Important disclaimer: Alexonic Tools is completely free to use. There is no charge, and we do not save tool inputs or generated results. We value customer privacy and keep building and fixing each day. Always verify important financial, payroll, legal, tax, business, or production-code results before relying on them. If you see an issue, need a tool, or require an update, send feedback to the developer.