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How to use CIDR for subnetting?

Created on 2025-02-24 15:25:17
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CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and for routing Internet Protocol packets. It's used for subnetting, which is the process of dividing an IP network into smaller subnetworks or subnets.

To use CIDR for subnetting, you first need to understand the basic structure of an IP address and its subnet mask. An IP address consists of 32 bits, typically represented in decimal form with dots separating every 8 bits (e.g., 192.168.1.1). The subnet mask also consists of 32 bits and is used to determine the network portion and the host portion of the IP address.

Here's how to use CIDR for subnetting:

Step-by-Step Guide:

  1. Determine the Original Network:

    • Start with an IP address and its subnet mask. For example, 192.168.1.0/24.

  2. Understand the CIDR Notation:

    • The /24 in 192.168.1.0/24 represents the number of bits used for the network portion. In this case, 24 bits are used for the network, and the remaining 8 bits are for host addresses.

  3. Calculate the Number of Hosts:

    • The formula to calculate the number of hosts in a subnet is 2^(32 - CIDR) - 2. The -2 accounts for the network address and the broadcast address.
    • For /24, the calculation is 2^(32-24) - 2 = 256 - 2 = 254 hosts.

  4. Subnetting:

    • To create smaller subnets, you need to borrow bits from the host portion of the IP address.
    • For example, if you want to create subnets with 64 hosts each, you need 6 bits for the host portion (2^6 - 2 = 62 hosts, but we use 6 bits to simplify).
    • This means you will use a /26 subnet mask (24 + 2 = 26).

  5. Assign Subnets:

    • Starting from 192.168.1.0/24, you can create subnets like:
      • 192.168.1.0/26 (Range: 192.168.1.0 - 192.168.1.63)
      • 192.168.1.64/26 (Range: 192.168.1.64 - 192.168.1.127)
      • And so on.

Example:

If you have a network 10.0.0.0/16 and you want to create subnets that can each support 256 hosts:

  1. Determine the New Subnet Mask:

    • For 256 hosts, you need 8 bits for the host portion (2^8 - 2 = 254 hosts).
    • The new subnet mask will be /24.

  2. Create Subnets:

    • You can create subnets like:
      • 10.0.0.0/24 (Range: 10.0.0.0 - 10.0.0.255)
      • 10.0.1.0/24 (Range: 10.0.1.0 - 10.0.1.255)
      • And continue this pattern.

Tencent Cloud Relevance:

When working with subnets in cloud environments like Tencent Cloud, you can easily create and manage subnets using their Virtual Private Cloud (VPC) service. Tencent Cloud allows you to define your own IP ranges and subnet masks, making it straightforward to implement CIDR-based subnetting for your network infrastructure.

By leveraging Tencent Cloud's VPC, you can ensure that your subnets are securely isolated and properly configured to meet your networking needs, whether for deploying applications, databases, or other services.