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website: Docs for all of the IP address calculation functions
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---
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page_title: cidrhost - Functions - Configuration Language
website: Docs for all of the IP address calculation functions
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description: |-
The cidrhost function calculates a full host IP address within a given
IP network address prefix.
---
# `cidrhost` Function
`cidrhost` calculates a full host IP address for a given host number within
a given IP network address prefix.
```hcl
cidrhost(prefix, hostnum)
```
`prefix` must be given in CIDR notation, as defined in
[RFC 4632 section 3.1](https://tools.ietf.org/html/rfc4632#section-3.1).
`hostnum` is a whole number that can be represented as a binary integer with
no more than the number of digits remaining in the address after the given
website: Additional explanation for cidrsubnet function Some users are not accustomed to thinking of IP addresses in a bitwise fashion, so the hope here is to give enough of an introduction to that way of thinking for the reader to understand what the "newbits" and "netnum" arguments represent.
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prefix. For more details on how this function interprets CIDR prefixes and
populates host numbers, see the worked example for
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[`cidrsubnet`](/language/functions/cidrsubnet).
website: Additional explanation for cidrsubnet function Some users are not accustomed to thinking of IP addresses in a bitwise fashion, so the hope here is to give enough of an introduction to that way of thinking for the reader to understand what the "newbits" and "netnum" arguments represent.
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Conventionally host number zero is used to represent the address of the
network itself and the host number that would fill all the host bits with
binary 1 represents the network's broadcast address. These numbers should
generally not be used to identify individual hosts except in unusual
situations, such as point-to-point links.
website: Docs for all of the IP address calculation functions
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This function accepts both IPv6 and IPv4 prefixes, and the result always uses
the same addressing scheme as the given prefix.
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.
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-> **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.
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## Examples
```
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> cidrhost("10.12.112.0/20", 16)
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10.12.112.16
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> cidrhost("10.12.112.0/20", 268)
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10.12.113.12
> cidrhost("fd00:fd12:3456:7890:00a2::/72", 34)
fd00:fd12:3456:7890::22
```
## Related Functions
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* [`cidrsubnet`](/language/functions/cidrsubnet) calculates a subnet address under a given
website: Docs for all of the IP address calculation functions
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network address prefix.