Subnet-Calculator.Pro
CIDR Calculator
...About the CIDR Calculator
This tool offers two modes for CIDR (Classless Inter-Domain Routing) calculations. The **Basic CIDR Details** mode provides a quick analysis of a single IP/CIDR block, showing key details like network ID, broadcast address, and host range. The **Advanced Subnetting** mode is a powerful tool for network administrators and engineers to plan the subdivision of a larger IP address space. It takes a larger network block and shows you all the possible smaller subnets you can create from it based on a new prefix length.
Use it to visualize your network allocations, plan for future growth, and ensure an efficient and organized IP addressing scheme. This is fundamental for tasks like route summarization, network segmentation, and creating Access Control Lists (ACLs).
A Guide to CIDR and Advanced IPv4 Subnetting
Efficiently divide your IPv4 address space with our advanced CIDR Calculator. This tool is designed for network administrators and students to visualize how a larger network block can be subdivided into multiple, equal-sized subnets. See a clear allocation table, understand the bit-level changes, and plan your network with precision. Whether you're breaking down a /16 into /24s or creating point-to-point links from a /24, this tool simplifies the process.
About the CIDR Calculator
This CIDR (Classless Inter-Domain Routing) Calculator is a powerful utility for network planning that goes beyond a basic subnet calculator. Its primary function is to demonstrate the process of subnetting—taking a larger IP address block and dividing it into a set of smaller, manageable, equal-sized subnets. It provides two modes: a 'Basic' mode for a quick analysis of a single CIDR block, and an 'Advanced Subnetting' mode to generate a complete allocation table. This is essential for planning network segmentation, preparing for IT certifications like the CCNA, and understanding the core principles of efficient IP address management.
How the Calculator Works and How to Interpret the Results
The tool is designed to be intuitive for both simple lookups and complex subnetting tasks.
Using the Calculator
- Select a Mode: Choose between 'Basic CIDR Details' for analyzing a single IP/CIDR entry, or 'Advanced Subnetting' for dividing a network.
- Advanced Subnetting Inputs:
- Base Network Address: Enter the network ID of the large block you wish to subdivide (e.g., `10.10.0.0`).
- Address Block Mask: Select the CIDR prefix of this large block (e.g., `/16`).
- Define New Subnet Size: You can define the size of your new, smaller subnets in two ways: by selecting the number of 'Subnet Bits' to borrow, or by directly choosing the 'New Subnet Mask' (e.g., `/24`). The fields are synchronized.
- Calculate: Click the "Calculate Subnets" button to generate the allocation plan.
Interpreting the Results
- Address Block Range: Shows the first and last IP addresses of the large parent network you are subdividing.
- Bit Usage Visualizer: This is a key feature for learning. It shows the 32 bits of an address, color-coded:
- n (Network): The bits belonging to the original, large network block.
- s (Subnet): The bits you 'borrowed' from the host portion to create your new subnets. The number of 's' bits determines how many subnets you can create (2^s).
- h (Host): The remaining bits, which determine how many hosts are available in each new, smaller subnet.
- Subnet Allocation Table: This table is the main output. It lists every new subnet that was created from the parent block, showing its Network Address, Usable Host Range, and Broadcast Address.
The Fundamentals of CIDR and Subnetting
CIDR (Classless Inter-Domain Routing) revolutionized IP addressing by replacing the rigid classful system (Class A, B, C) with a more flexible and efficient method. It is the foundation of the modern internet's scalability.
From Classful to Classless
The original classful system had significant drawbacks, primarily its inefficient allocation of IP addresses. A Class A network, for example, contained over 16 million addresses, far too large for any single organization and leading to immense waste. CIDR introduced the concept of a variable-length prefix, allowing network blocks to be sized precisely as needed.
| CIDR Prefix | Subnet Mask | Total Addresses | Usable Hosts | Common Use Case |
|---|---|---|---|---|
/30 |
255.255.255.252 | 4 | 2 | Point-to-Point WAN Links |
/27 |
255.255.255.224 | 32 | 30 | Small LAN or VLAN |
/24 |
255.255.255.0 | 256 | 254 | Standard LAN, Small Office |
/16 |
255.255.0.0 | 65,536 | 65,534 | Large Enterprise Network Block |
/8 |
255.0.0.0 | 16,777,216 | 16,777,214 | Very Large Enterprise or ISP Block |
The Process of Subnetting
Subnetting is the process of taking a single network block and dividing it into multiple smaller blocks. This is done by extending the network prefix—essentially, 'borrowing' bits from the host portion of the address to create a new subnet portion.
Example: Subdividing a /24 Network
Imagine you have the network block 192.168.10.0/24. You need to create four separate networks for different departments, each needing around 50 hosts.
- Determine Required Subnet Size: To support 50 hosts, you need at least 50 + 2 (for network and broadcast addresses) = 52 total addresses. The next power of two is 64. A block of 64 addresses requires 6 host bits (2^6 = 64).
- Calculate New Prefix: Since you need 6 host bits, the new network prefix will be 32 - 6 = 26. So, your new subnets will be /26.
- Calculate Number of Subnets: The original prefix was /24, and the new prefix is /26. You borrowed 2 bits (26 - 24 = 2) for subnetting. This allows you to create 2^2 = 4 new subnets.
- Use the Calculator: By entering `192.168.10.0` as the base address, `/24` as the block mask, and `/26` as the new mask, the tool will generate the following allocation table:
- Subnet 1: `192.168.10.0/26`
- Subnet 2: `192.168.10.64/26`
- Subnet 3: `192.168.10.128/26`
- Subnet 4: `192.168.10.192/26`
A well-designed CIDR and subnetting plan is the foundation of a scalable and secure network. It enables effective route summarization, which reduces the load on routers, and provides logical separation for applying security policies. For any business network, planning this structure thoughtfully is a critical task for a network professional.
Frequently Asked Questions about CIDR
What is CIDR?
CIDR (Classless Inter-Domain Routing) is the modern method for allocating IP addresses and routing Internet Protocol packets. It replaces the old classful system (Class A, B, C) with a more flexible notation (e.g., 192.168.1.0/24), where the number after the slash indicates the number of bits in the network prefix.
How do I use this CIDR Calculator?
For advanced subnetting, enter a base IP and its block mask (e.g., 10.0.0.0 and /16). Then, choose a new, larger subnet mask (e.g., /24) to see how the /16 block is divided into smaller /24 subnets. The 'Basic' mode gives you details for a single CIDR block.
Why is CIDR important?
CIDR is important because it allows for efficient use of the limited IPv4 address space. It enables route summarization (supernetting) to reduce the size of global routing tables and allows for flexible subnetting (VLSM) to match network sizes to actual needs.
What does the /24 in 192.168.1.0/24 mean?
The /24 is the CIDR prefix length. It means that the first 24 bits of the IP address are the network portion, and the remaining 8 bits (32 - 24 = 8) are for the host portion. This corresponds to a subnet mask of 255.255.255.0.
How many hosts are in a /24 network?
A /24 network has 8 host bits, which allows for 2^8 = 256 total addresses. After reserving one for the network address and one for the broadcast address, there are 254 usable host addresses.
What is the difference between subnetting and supernetting?
Subnetting is the process of dividing a large network block into multiple smaller sub-networks. Supernetting (or route summarization) is the opposite: combining multiple smaller, contiguous network blocks into a single, larger route advertisement. You can calculate supernets with our Supernet Calculator.
What are subnet bits?
Subnet bits are the bits 'borrowed' from the original host portion of an address block to create new, smaller subnets. For example, if you divide a /24 network into /26 subnets, you have used 2 subnet bits (26 - 24 = 2), creating 2^2 = 4 new subnets.
How does this tool help with network planning?
This tool allows you to visualize how a large IP address allocation can be broken down. This is essential for planning network segments for different departments, services, or locations while ensuring each segment has the correct number of addresses and they don't overlap.
What is a subnet mask?
A subnet mask is a 32-bit number that separates the network portion of an IP address from the host portion. In binary, the 1s represent the network part and the 0s represent the host part.
How do I convert CIDR to a subnet mask?
To convert a CIDR prefix to a subnet mask, you write out the number of 1s indicated by the prefix, and the rest of the 32 bits are 0s. For example, /24 is 24 ones followed by 8 zeros, which in binary is 11111111.11111111.11111111.00000000, or 255.255.255.0 in decimal.
What is the bit usage visualizer (n.s.h)?
The bit usage visualizer shows the 32 bits of an address. 'n' represents the bits of the original network block. 's' represents the bits you have borrowed for subnetting. 'h' represents the remaining bits available for hosts in each new subnet.
What was the problem with classful addressing?
The old classful system (Class A, B, C) was very rigid. It allocated huge blocks of addresses (e.g., a Class A had over 16 million addresses) that were often wasted. CIDR allows for flexible block sizes that match the actual needs of an organization.
Can I divide a /24 into /30 subnets?
Yes. A /24 network has 256 addresses. A /30 subnet has 4 addresses. You can create 64 individual /30 subnets from a single /24 block. This is commonly done to create point-to-point links between routers.
What is a broadcast address?
The broadcast address is the last IP address in a subnet. Packets sent to this address are delivered to all hosts on that specific subnet. It's used by protocols like ARP and DHCP.
What is a network address?
The network address (or network ID) is the first IP address in a subnet. It represents the network itself and cannot be assigned to a specific device.
Why is there a 256 subnet display limit in this tool?
Displaying thousands of subnets can be slow and unmanageable in a web browser. The 256 subnet limit is for performance and usability reasons, while still allowing for common subnetting scenarios.
What is a usable host range?
The usable host range is the block of IP addresses within a subnet that can be assigned to devices like computers and printers. It is the range between the network address and the broadcast address.
How many subnets can I get from a /16 block if I use a /24 mask?
You are using 8 subnet bits (24 - 16 = 8). This allows you to create 2^8 = 256 individual /24 subnets from the /16 block.
What is a wildcard mask?
A wildcard mask is the inverse of a subnet mask and is primarily used in Access Control Lists (ACLs). Where a subnet mask has a '1', a wildcard mask has a '0', and vice-versa. You can calculate these with our Wildcard Mask Calculator.
What is VLSM?
VLSM (Variable Length Subnet Masking) is a technique that allows you to divide an IP address space into subnets of different sizes. This tool creates equal-sized subnets, but you can plan for VLSM with our dedicated VLSM Calculator.
What is a private IP address?
A private IP address is an address from a range reserved by RFC 1918 for use within internal networks (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16). They are not routable on the public internet.
How does CIDR relate to IPv6?
IPv6 was designed from the ground up to use CIDR notation. All IPv6 addresses are written with a prefix length (e.g., /64, /48) to define the network portion. You can work with IPv6 addresses using our IPv6 Calculator.
Can any IP address be used as a base?
While you can enter any IP address in the tool, for a real network, the base address you subnet should be the network address of the block you have been allocated (e.g., use 10.10.0.0 for a 10.10.0.0/16 block, not 10.10.5.1).
What does 'route aggregation' mean?
Route aggregation, also known as route summarization or supernetting, is the process of combining multiple smaller network routes into a single larger one. This is a key benefit of a well-planned CIDR scheme, as it helps keep routing tables on the internet small and efficient.
Do I need a network professional for subnetting?
For a home network, basic subnetting is straightforward. For any business network, designing a CIDR and subnetting plan is a critical task. A network professional should be involved to ensure the plan is scalable, secure, and supports the organization's routing and security policies.