Subnetting is the practice of dividing a larger IP network into smaller, more manageable sub-networks (subnets). It helps improve network performance by reducing broadcast traffic, enhances security by isolating network segments, and makes more efficient use of IP address space. Each subnet has its own network address, broadcast address, and range of usable host addresses.

What is a subnet mask?

A subnet mask is a 32-bit number that defines which portion of an IP address identifies the network and which portion identifies the host. For example, 255.255.255.0 (/24) means the first 24 bits are the network portion and the last 8 bits are the host portion, allowing 254 usable host addresses. Common subnet masks include /8 (255.0.0.0), /16 (255.255.0.0), and /24 (255.255.255.0).

What is the difference between network address and broadcast address?

The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself. The broadcast address is the last address (all host bits set to 1) and is used to send packets to all hosts in the subnet. Neither can be assigned to a device, which is why a /24 subnet has 256 total addresses but only 254 usable host addresses.

How do I choose the right subnet size?

Choose a subnet size based on how many hosts you need. Count your devices and add room for growth, then pick the smallest CIDR prefix that fits. For example: /28 gives 14 hosts (small office), /24 gives 254 hosts (typical LAN), /20 gives 4094 hosts (campus network). Always leave some room for future expansion.

Subnet Calculator - Free IP Subnet & CIDR Calculator

What is IP Subnetting?

IP subnetting is the process of dividing a large IP network into smaller, logical sub-networks. Every IPv4 address is a 32-bit number, split into a network portion and a host portion. The subnet mask (or CIDR prefix) determines where that split happens. Subnetting lets network administrators control traffic flow, improve security through isolation, and use IP address space more efficiently.

CIDR (Classless Inter-Domain Routing) notation uses a slash followed by a number to indicate the subnet mask. For example, 192.168.1.0/24 means the first 24 bits are the network address, leaving 8 bits for host addresses (256 total, 254 usable). The smaller the CIDR number, the larger the network.

How to Use This Tool

1. Enter an IP address - Type any IPv4 address (e.g., 192.168.1.0)
2. Select a CIDR prefix - Choose the subnet size from /8 to /32
3. Click Calculate - See the network address, broadcast, host range, masks, and host count

CIDR Quick Reference

CIDR	Subnet Mask	Usable Hosts
/8	255.0.0.0	16,777,214
/16	255.255.0.0	65,534
/20	255.255.240.0	4,094
/24	255.255.255.0	254
/25	255.255.255.128	126
/26	255.255.255.192	62
/27	255.255.255.224	30
/28	255.255.255.240	14
/29	255.255.255.248	6
/30	255.255.255.252	2

Common Use Cases

Cloud VPC planning - Size your AWS VPC, Azure VNet, or GCP subnets correctly before deployment
Network segmentation - Separate servers, databases, and application tiers into different subnets
Firewall rules - Calculate CIDR ranges for allow/deny rules in security groups
IP address management - Plan address allocation for office networks and data centers
Kubernetes networking - Configure pod and service CIDR ranges for cluster networking

Frequently Asked Questions

What is the difference between a subnet mask and a wildcard mask?

A subnet mask and wildcard mask are inverses of each other. The subnet mask has 1s for the network portion and 0s for the host portion (e.g., 255.255.255.0). The wildcard mask has 0s for the network portion and 1s for the host portion (e.g., 0.0.0.255). Wildcard masks are used in Cisco ACLs and OSPF configurations, while subnet masks are used everywhere else. To convert between them, subtract the subnet mask from 255.255.255.255.

Why does a /24 subnet only have 254 usable hosts instead of 256?

A /24 subnet has 256 total addresses (2⁸), but two are reserved: the first address is the network address (identifies the subnet itself) and the last address is the broadcast address (used to send packets to all hosts). Neither can be assigned to a device, so 256 - 2 = 254 usable host addresses. The exception is /31 (point-to-point links) and /32 (single host), which have special rules.

What are the private IP address ranges?

RFC 1918 defines three private IP ranges that are not routable on the public internet: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255, ~16.7 million addresses), 172.16.0.0/12 (172.16.0.0 - 172.31.255.255, ~1 million addresses), and 192.168.0.0/16 (192.168.0.0 - 192.168.255.255, ~65,000 addresses). Most home routers use 192.168.x.x, while cloud providers and enterprises typically use 10.x.x.x.

How do I choose subnet sizes for AWS VPCs?

AWS recommends a VPC CIDR between /16 and /28. A common pattern is /16 for the VPC (65,534 IPs) with /24 subnets for each availability zone. For example: public subnet /24 in each AZ for load balancers, private subnet /24 for application servers, and a separate /24 for databases. Remember that AWS reserves 5 IPs per subnet (first 4 + last 1), so a /24 gives 251 usable IPs in AWS.

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Subnet Calculator

What is IP Subnetting?

How to Use This Tool

CIDR Quick Reference

Common Use Cases

Frequently Asked Questions

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