c23a7fce4e
Go 1.17 includes a breaking change to both net.ParseIP and net.ParseCIDR functions to reject IPv4 address octets written with leading zeros. Our use of these functions as part of the various CIDR functions in the Terraform language doesn't have the same security concerns that the Go team had in evaluating this change to the standard library, and so we can't justify an exception to our v1.0 compatibility promises on the same sort of security grounds that the Go team used to justify their compatibility exception. For that reason, we'll now use our own fork of the Go library functions which has the new check disabled in order to preserve the prior behavior. We're taking this path, rather than pre-normalizing the IP address before calling into the standard library, because an additional normalization layer would be entirely new code and additional complexity, whereas this fork is relatively minor in terms of code size and avoids any significant changes to our own calls to these functions. Thanks to the Kubernetes team for their prior work on carving out a subset of the "net" package for their similar backward-compatibility concern. Our "ipaddr" package here is a lightly-modified fork of their fork, with only the comments changed to talk about Terraform instead of Kubernetes. This fork is not intended for use in any other future feature implementations, because they wouldn't be subject to the same compatibility constraints as our existing functions. We will use these forked implementations for new callers only if consistency with the behavior of the existing functions is a key requirement.
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| language | cidrsubnets - Functions - Configuration Language | docs-funcs-ipnet-cidrsubnets | The cidrsubnets function calculates a sequence of consecutive IP address ranges within a particular CIDR prefix. |
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.
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 Terraform module hashicorp/subnets/cidr
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
cidrhostcalculates the IP address for a single host within a given network address prefix.cidrnetmaskconverts an IPv4 network prefix in CIDR notation into netmask notation.cidrsubnetcalculates a single subnet address, allowing you to specify its network number.