CIDR / Subnet Calculator
Enter any IPv4 CIDR block and instantly see the network range, host count, subnet mask, and split into child subnets.
CIDR block
About this tool
A free IPv4 subnet calculator for network engineers and developers. Enter any CIDR block (e.g. 192.168.1.0/24) and see the network address, broadcast address, first and last usable hosts, total host count, subnet mask, wildcard mask, and binary representation. The subnet splitter lets you divide the block into smaller child subnets.
How to use it
Quick steps to get the most out of this utility.
- 1
Enter a CIDR block
Type an IPv4 CIDR like 192.168.1.0/24, or click one of the preset examples (10.0.0.0/8, 172.16.0.0/12, etc.).
- 2
Read the subnet details
The network address, broadcast address, first and last usable host, and host counts appear immediately.
- 3
Check the masks
View the dotted-decimal subnet mask (255.255.255.0), wildcard mask (0.0.0.255), and binary representation.
- 4
Split into subnets
Enter a larger prefix (e.g. /26) in the splitter to see the child networks within your block.
Understanding CIDR notation
CIDR stands for Classless Inter-Domain Routing. Before CIDR (before 1993), IP addresses were divided into fixed classes: Class A gave you 16 million hosts, Class B 65,000, and Class C 254 — nothing in between. CIDR replaced this with a flexible prefix length (/8 through /32) that lets you allocate exactly the number of addresses you need.
A /24 block has 256 total addresses. Subtract 2 (network + broadcast) and you get 254 usable hosts — plenty for a small office or a single VPC subnet. For a container cluster that needs hundreds of pods, a /22 (1022 usable) or /21 (2046 usable) is common. Cloud providers like AWS and Azure use CIDR heavily in their VPC and virtual network configuration.
Worked example: 10.10.10.0/28
A /28 block has a 28-bit network prefix, leaving 4 bits for hosts. That's 2⁴ = 16 total addresses. Subtract 2 for network (10.10.10.0) and broadcast (10.10.10.15) and you have 14 usable hosts — perfect for a small DMZ segment or a point-to-point link with room to spare. The subnet mask is 255.255.255.240 and the wildcard is 0.0.0.15.
Private ranges to memorise
10.0.0.0/8— Class A private, ~16.7M addresses172.16.0.0/12— Class B private, ~1M addresses192.168.0.0/16— Class C private, 65,536 addresses100.64.0.0/10— Carrier-grade NAT (CGNAT), RFC 6598169.254.0.0/16— Link-local (APIPA when DHCP fails)
Frequently asked questions
What does /24 mean in CIDR notation?+
The /24 is the prefix length — it means the first 24 bits of the address are the network portion, leaving 8 bits for host addresses. A /24 network has 256 total addresses (2^8), of which 254 are usable for hosts (network and broadcast addresses are reserved).
What are the network and broadcast addresses?+
The network address is the first address in the block (all host bits zero). The broadcast address is the last address (all host bits one). Neither can be assigned to a host. For a /24 block like 192.168.1.0/24, the network is 192.168.1.0 and the broadcast is 192.168.1.255.
What is a subnet mask?+
A subnet mask is a 32-bit number with all 1s in the network portion and all 0s in the host portion. For /24 it is 255.255.255.0. Routers use the subnet mask to determine whether a destination IP is on the local subnet or needs to be forwarded.
What is a wildcard mask?+
A wildcard mask is the bitwise inverse of the subnet mask. For 255.255.255.0 the wildcard is 0.0.0.255. Wildcard masks are commonly used in Cisco ACLs and OSPF network statements.
What are private IP ranges?+
RFC 1918 defines three private ranges: 10.0.0.0/8 (16 million addresses), 172.16.0.0/12 (1 million addresses), and 192.168.0.0/16 (65,536 addresses). These are not routed on the public Internet and are used in private networks and VPNs.
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