cidrsubnets Function

cidrsubnets calculates a sequence of consecutive IP address ranges within a particular CIDR prefix.

cidrsubnets(prefix, newbits...)

prefix must be given in CIDR notation, as defined in RFC 4632 section 3.1 (opens in a new tab).

The remaining arguments, indicated as newbits above, each specify the number of additional network prefix bits for one returned address range. The return value is therefore a list with one element per newbits argument, each a string containing an address range in CIDR notation.

For more information on IP addressing concepts, see the documentation for the related function cidrsubnet. cidrsubnet calculates a single subnet address within a prefix while allowing you to specify its subnet number, while cidrsubnets can calculate many at once, potentially of different sizes, and assigns subnet numbers automatically.

When using this function to partition an address space as part of a network address plan, you must not change any of the existing arguments once network addresses have been assigned to real infrastructure, or else later address assignments will be invalidated. However, you can append new arguments to existing calls safely, as long as there is sufficient address space available.

This function accepts both IPv6 and IPv4 prefixes, and the result always uses the same addressing scheme as the given prefix.

:::note As a historical accident, this function interprets IPv4 address octets that have leading zeros as decimal numbers, which is contrary to some other systems which interpret them as octal. We have preserved this behavior for backward compatibility, but recommend against relying on this behavior. :::

:::note The hashicorp/subnets/cidr module (opens in a new tab) wraps cidrsubnets to provide additional functionality for assigning symbolic names to your networks and skipping prefixes for obsolete allocations. Its documentation includes usage examples for several popular cloud virtual network platforms. :::

Examples

> cidrsubnets("10.1.0.0/16", 4, 4, 8, 4)
[
  "10.1.0.0/20",
  "10.1.16.0/20",
  "10.1.32.0/24",
  "10.1.48.0/20",
]

> cidrsubnets("fd00:fd12:3456:7890::/56", 16, 16, 16, 32)
[
  "fd00:fd12:3456:7800::/72",
  "fd00:fd12:3456:7800:100::/72",
  "fd00:fd12:3456:7800:200::/72",
  "fd00:fd12:3456:7800:300::/88",
]

You can use nested cidrsubnets calls with for expressions to concisely allocate groups of network address blocks:

> [for cidr_block in cidrsubnets("10.0.0.0/8", 8, 8, 8, 8) : cidrsubnets(cidr_block, 4, 4)]
[
  [
    "10.0.0.0/20",
    "10.0.16.0/20",
  ],
  [
    "10.1.0.0/20",
    "10.1.16.0/20",
  ],
  [
    "10.2.0.0/20",
    "10.2.16.0/20",
  ],
  [
    "10.3.0.0/20",
    "10.3.16.0/20",
  ],
]

Related Functions

• cidrhost calculates the IP address for a single host within a given network address prefix.
• cidrnetmask converts an IPv4 network prefix in CIDR notation into netmask notation.
• cidrsubnet calculates a single subnet address, allowing you to specify its network number.