What is CIDR?

Classless Inter-Domain Routing (CIDR) is a range of IP addresses a network uses. A CIDR address looks like a normal IP address, except that it ends with a slash followed by a number. The number after the slash represents the number of addresses in the range.

Here’s an example CIDR IP address in IPv4:

192.0.2.0/24

Because IPv4 has a 32-bit address space, the 24-bit prefix in the preceding example means that the address range is the 8 bits (256 addresses) after 192.0.2.0.

And here’s an example in IPv6:

2001:db8::/32

IPv6 has a 128-bit address range, so the 32-bit network prefix refers to 96 bits worth of addresses following 2001:db8::, about 76 octillion addresses.

CIDR notation in the Cloud Control Panel

If you’re using the Cloud Control Panel to create a Cloud Network, you don’t need to worry about calculating the IP address. The system automatically populates the CIDR field with an IP block on the Rackspace Network. You need only to supply a name for the network and click Create Network.

For complete instructions on creating a Cloud Network by using the Cloud Control Panel, see Create an Isolated Cloud Network and attach it to a server.

CIDR notation in the Cloud Networks API

If you’re using the Cloud Networks API to create a network, you must specify the CIDR and a name for the new network.

The nova client command looks like the following example:

nova network-create <network_label> <cidr>

In this case, network_label is the name of the network, such as my_new_network, and cidr is the IP block from which to allocate, such as 172.16.0.0/24 or 2001:DB8::/64.

The following example shows a cURL command that creates the network:

curl -s https://dfw.servers.api.rackspacecloud.com/v2/010101/os-networksv2 <br>
         -X POST -H "X-Auth-Project-Id: 010101" -H "Content-Type: application/json" <br>
         -H "Accept: application/json" <br>
         -H "X-Auth-Token: 574c5763-86ab-431e-8b6c-5f7cf84c30ca" <br>
         -d '{"network": {"cidr": "192.168.0.0/24", "label": "superprivate"}}' | python -m json.tool

The JSON-format response would look similar to this:

{
 "network": {
 "cidr": "192.168.0.0/24",
 "id": "aaa77608-bae2-4eed-840e-896a7dd345d4",
 "label": "superprivate"
 }
}

Note: To help calculate a subnet, you might use a tool like this: https://www.subnet-calculator.com/cidr.php

For more information on using the Cloud Networks API see the Rackspace Cloud Networks Developers Guide.

More Information on Cloud Networks

Attach an Isolated Network to a New Cloud Server

Attach an Isolated Network to an Existing Cloud Server

Removing Networks from a Cloud Server

Cloud Networks Developer Guide

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