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website: Docs for all of the IP address calculation functions
2018-05-13 23:11:13 +02:00
---
migrate docs to mdx
2021-12-15 03:41:17 +01:00
page_title: cidrnetmask - Functions - Configuration Language
website: Docs for all of the IP address calculation functions
2018-05-13 23:11:13 +02:00
description: |-
The cidrnetmask function converts an IPv4 address prefix given in CIDR
notation into a subnet mask address.
---
# `cidrnetmask` Function
`cidrnetmask` converts an IPv4 address prefix given in CIDR notation into
a subnet mask address.
```hcl
cidrnetmask(prefix)
```
`prefix` must be given in IPv4 CIDR notation, as defined in
[RFC 4632 section 3.1](https://tools.ietf.org/html/rfc4632#section-3.1).
The result is a subnet address formatted in the conventional dotted-decimal
IPv4 address syntax, as expected by some software.
CIDR notation is the only valid notation for IPv6 addresses, so `cidrnetmask`
produces an error if given an IPv6 address.
lang/funcs: Preserve IP address leading zero behavior from Go 1.16 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.
2021-08-17 20:30:18 +02:00
-> **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.
website: Docs for all of the IP address calculation functions
2018-05-13 23:11:13 +02:00
## Examples
```
> cidrnetmask("172.16.0.0/12")
255.240.0.0
```