Skip to main content

CIDR / Subnet Calculator

A tool to calculate network information using IP addresses and CIDR notation. Automatically calculates subnet masks, available host counts, IP ranges, and more.

Key Features​

  • IP address and CIDR notation input
  • Automatic subnet mask calculation
  • Network address and broadcast address calculation
  • Display available IP range
  • Calculate total and usable host counts
  • Identify IP class and type (Private/Public)
  • Binary representation provided
  • 5 example templates

What is CIDR Notation?​

CIDR (Classless Inter-Domain Routing) is a method for efficiently allocating and routing IP addresses.

Notation Format​

IPAddress/SubnetMaskBits
Example: 192.168.1.0/24

Common CIDR Notations​

CIDRSubnet MaskUsable HostsPurpose
/8255.0.0.016,777,214Large networks (Class A)
/16255.255.0.065,534Medium networks (Class B)
/24255.255.255.0254Small networks (Class C)
/28255.255.255.24014Very small networks
/30255.255.255.2522Point-to-Point links
/32255.255.255.2550Single host

Terminology​

Network Address​

The first IP address of a subnet, used to identify the network itself.

Broadcast Address​

The last IP address of a subnet, used to send messages to all hosts in the network.

Usable IP Range​

The range of IP addresses that can be assigned to hosts, excluding the network and broadcast addresses.

Wildcard Mask​

The inverse of the subnet mask, primarily used in ACL (Access Control List) configurations.

Usage Examples​

Small Office Network (/24)​

IP Address: 192.168.1.0
CIDR: /24
Subnet Mask: 255.255.255.0
Usable IPs: 192.168.1.1 ~ 192.168.1.254
Total of 254 usable hosts

Medium Enterprise Network (/16)​

IP Address: 172.16.0.0
CIDR: /16
Subnet Mask: 255.255.0.0
Usable IPs: 172.16.0.1 ~ 172.16.255.254
Total of 65,534 usable hosts

Very Small Network (/28)​

IP Address: 192.168.1.16
CIDR: /28
Subnet Mask: 255.255.255.240
Usable IPs: 192.168.1.17 ~ 192.168.1.30
Total of 14 usable hosts

Private IP Address Ranges​

Private IPs are not directly routed on the Internet and are only used in internal networks:

  • Class A: 10.0.0.0 ~ 10.255.255.255 (/8)
  • Class B: 172.16.0.0 ~ 172.31.255.255 (/12)
  • Class C: 192.168.0.0 ~ 192.168.255.255 (/16)

Tips​

  1. Network Design: Choose an appropriate CIDR considering the required number of hosts.

  2. Subnet Division: Divide large networks into smaller subnets for efficient management.

  3. Security Policy: Apply different security policies to each network by dividing subnets.

  4. IP Allocation Planning: Check the available IP range and systematically assign IPs to hosts.

  5. Router Configuration: Use CIDR information to configure router routing tables.

Practical Use Cases​

AWS VPC Design​

VPC CIDR: 10.0.0.0/16
Public Subnet 1: 10.0.1.0/24 (254 hosts)
Public Subnet 2: 10.0.2.0/24 (254 hosts)
Private Subnet 1: 10.0.10.0/24 (254 hosts)
Private Subnet 2: 10.0.11.0/24 (254 hosts)

Office Network Segmentation​

Headquarters: 192.168.1.0/24
Branch 1: 192.168.2.0/24
Branch 2: 192.168.3.0/24
VPN: 192.168.100.0/24