- The OpenTofu Language
- Built-in Functions
- cidrsubnet

`cidrsubnet`

Function

`cidrsubnet`

calculates a subnet address within given IP network address prefix.

`prefix`

must be given in CIDR notation, as defined in
RFC 4632 section 3.1.

`newbits`

is the number of additional bits with which to extend the prefix.
For example, if given a prefix ending in `/16`

and a `newbits`

value of
`4`

, the resulting subnet address will have length `/20`

.

`netnum`

is a whole number that can be represented as a binary integer with
no more than `newbits`

binary digits, which will be used to populate the
additional bits added to the prefix.

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

Unlike the related function `cidrsubnets`

, `cidrsubnet`

allows you to give a specific network number to use. `cidrsubnets`

can allocate
multiple network addresses at once, but numbers them automatically starting
with zero.

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.

## Examples

## Netmasks and Subnets

Using `cidrsubnet`

requires familiarity with some network addressing concepts.

The most important idea is that an IP address (whether IPv4 or IPv6) is fundamentally constructed from binary digits, even though we conventionally represent it as either four decimal octets (for IPv4) or a sequence of 16-bit hexadecimal numbers (for IPv6).

Taking our example above of `cidrsubnet("10.1.2.0/24", 4, 15)`

, the function
will first convert the given IP address string into an equivalent binary
representation:

The `/24`

at the end of the prefix string specifies that the first 24
bits -- or, the first three octets -- of the address identify the network
while the remaining bits (32 - 24 = 8 bits in this case) identify hosts
within the network.

The CLI tool `ipcalc`

is useful for
visualizing CIDR prefixes as binary numbers. We can confirm the conversion
above by providing the same prefix string to `ipcalc`

:

This gives us some additional information but also confirms (using a slightly different notation) the conversion from decimal to binary and shows the range of possible host addresses in this network.

While `cidrhost`

allows calculating single host IP addresses,
`cidrsubnet`

on the other hand creates a new network prefix *within* the given
network prefix. In other words, it creates a subnet.

When we call `cidrsubnet`

we also pass two additional arguments: `newbits`

and
`netnum`

. `newbits`

decides how much longer the resulting prefix will be in
bits; in our example here we specified `4`

, which means that the resulting
subnet will have a prefix length of 24 + 4 = 28 bits. We can imagine these
bits breaking down as follows:

Four of the eight bits that were originally the "host number" are now being repurposed as the subnet number. The network prefix no longer falls on an exact octet boundary, so in effect we are now splitting the last decimal number in the IP address into two parts, using half of it to represent the subnet number and the other half to represent the host number.

The `netnum`

argument then decides what number value to encode into those
four new subnet bits. In our current example we passed `15`

, which is
represented in binary as `1111`

, allowing us to fill in the `XXXX`

segment
in the above:

To convert this back into normal decimal notation we need to recombine the
two portions of the final octet. Converting `11110000`

from binary to decimal
gives 240, which can then be combined with our new prefix length of 28 to
produce the result `10.1.2.240/28`

. Again we can pass this prefix string to
`ipcalc`

to visualize it:

The new subnet has four bits available for host numbering, which means
that there are 14 host addresses available for assignment once we subtract
the network's own address and the broadcast address. You can thus use
`cidrhost`

function to calculate those host addresses by
providing it a value between 1 and 14:

For more information on CIDR notation and subnetting, see Classless Inter-domain Routing.

## 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.`cidrsubnets`

can allocate multiple consecutive addresses under a prefix at once, numbering them automatically.