CIDR Calculator: Convert IP Address to CIDR Notation

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and its associated network mask. It consists of an IP address followed by a slash and the number of bits in the network mask (e.g., 192.168.1.0/24).

How do I calculate CIDR from a subnet mask?

To calculate CIDR from a subnet mask:

1. Convert each octet of the subnet mask to binary
2. Count the number of consecutive 1s in the binary representation
3. The total count is the CIDR suffix
4. Example: 255.255.255.0 in binary is 11111111.11111111.11111111.00000000 (24 ones)

What's the difference between network and broadcast addresses?

The network address is the first address in a subnet and identifies the network itself. The broadcast address is the last address in a subnet and is used to send data to all devices on that network.

Why can't I use the first and last addresses in a subnet?

The first address is reserved as the network identifier, and the last address is reserved for broadcast traffic. These addresses cannot be assigned to individual hosts.

How do I determine the number of usable hosts in a subnet?

The formula is: Usable hosts = 2^(32 - CIDR) - 2. For example, a /24 network has 2^(32-24) - 2 = 256 - 2 = 254 usable hosts.

What is the purpose of subnetting?

Subnetting helps with:

• Reducing network congestion by limiting broadcast domains
• Improving security by isolating network segments
• Optimizing IP address allocation
• Improving routing efficiency

What is VLSM (Variable Length Subnet Masking)?

VLSM is the process of subnetting a subnet to create networks of different sizes, allowing for more efficient use of IP address space.

How does CIDR differ from traditional classful networking?

Classful networking used fixed classes (A, B, C) with predetermined network sizes. CIDR allows for arbitrary network sizes, enabling more flexible and efficient address allocation.

What is a private IP address range?

Private IP ranges (not routable on the public internet) are:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

How do I choose the right subnet size?

Consider:

• Current number of hosts
• Expected growth
• Network segmentation needs
• Broadcast domain size

What is supernetting?

Supernetting is the opposite of subnetting - combining multiple smaller networks into a larger one to reduce routing table entries.

How do IPv6 addresses use CIDR notation?

IPv6 uses CIDR similarly to IPv4, but with much larger numbers (e.g., 2001:db8::/32). The principles are the same, but the address space is vastly larger.