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168 lines
6.8 KiB
Plaintext
168 lines
6.8 KiB
Plaintext
---
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layout: docs
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page_title: cidrsubnet - Functions - Configuration Language
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description: |-
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The cidrsubnet function calculates a subnet address within a given IP network
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address prefix.
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---
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# `cidrsubnet` Function
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`cidrsubnet` calculates a subnet address within given IP network address prefix.
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```hcl
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cidrsubnet(prefix, newbits, netnum)
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```
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`prefix` must be given in CIDR notation, as defined in
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[RFC 4632 section 3.1](https://tools.ietf.org/html/rfc4632#section-3.1).
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`newbits` is the number of additional bits with which to extend the prefix.
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For example, if given a prefix ending in `/16` and a `newbits` value of
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`4`, the resulting subnet address will have length `/20`.
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`netnum` is a whole number that can be represented as a binary integer with
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no more than `newbits` binary digits, which will be used to populate the
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additional bits added to the prefix.
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This function accepts both IPv6 and IPv4 prefixes, and the result always uses
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the same addressing scheme as the given prefix.
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Unlike the related function [`cidrsubnets`](/nomad/docs/reference/hcl2/functions/ipnet/cidrsubnets), `cidrsubnet`
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allows you to give a specific network number to use. `cidrsubnets` can allocate
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multiple network addresses at once, but numbers them automatically starting
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with zero.
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## Examples
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```shell-session
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> cidrsubnet("172.16.0.0/12", 4, 2)
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172.18.0.0/16
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> cidrsubnet("10.1.2.0/24", 4, 15)
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10.1.2.240/28
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> cidrsubnet("fd00:fd12:3456:7890::/56", 16, 162)
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fd00:fd12:3456:7800:a200::/72
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```
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## Netmasks and Subnets
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Using `cidrsubnet` requires familiarity with some network addressing concepts.
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The most important idea is that an IP address (whether IPv4 or IPv6) is
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fundamentally constructed from binary digits, even though we conventionally
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represent it as either four decimal octets (for IPv4) or a sequence of 16-bit
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hexadecimal numbers (for IPv6).
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Taking our example above of `cidrsubnet("10.1.2.0/24", 4, 15)`, the function
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will first convert the given IP address string into an equivalent binary
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representation:
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```text
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10 . 1 . 2 . 0
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00001010 00000001 00000010 | 00000000
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network | host
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```
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The `/24` at the end of the prefix string specifies that the first 24
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bits -- or, the first three octets -- of the address identify the network
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while the remaining bits (32 - 24 = 8 bits in this case) identify hosts
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within the network.
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The CLI tool [`ipcalc`](https://gitlab.com/ipcalc/ipcalc) is useful for
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visualizing CIDR prefixes as binary numbers. We can confirm the conversion
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above by providing the same prefix string to `ipcalc`:
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```shell-session
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$ ipcalc 10.1.2.0/24
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Address: 10.1.2.0 00001010.00000001.00000010. 00000000
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Netmask: 255.255.255.0 = 24 11111111.11111111.11111111. 00000000
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Wildcard: 0.0.0.255 00000000.00000000.00000000. 11111111
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=>
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Network: 10.1.2.0/24 00001010.00000001.00000010. 00000000
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HostMin: 10.1.2.1 00001010.00000001.00000010. 00000001
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HostMax: 10.1.2.254 00001010.00000001.00000010. 11111110
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Broadcast: 10.1.2.255 00001010.00000001.00000010. 11111111
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Hosts/Net: 254 Class A, Private Internet
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```
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This gives us some additional information but also confirms (using a slightly
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different notation) the conversion from decimal to binary and shows the range
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of possible host addresses in this network.
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While [`cidrhost`](/nomad/docs/reference/hcl2/functions/ipnet/cidrhost) allows calculating single host IP addresses,
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`cidrsubnet` on the other hand creates a new network prefix _within_ the given
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network prefix. In other words, it creates a subnet.
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When we call `cidrsubnet` we also pass two additional arguments: `newbits` and
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`netnum`. `newbits` decides how much longer the resulting prefix will be in
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bits; in our example here we specified `4`, which means that the resulting
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subnet will have a prefix length of 24 + 4 = 28 bits. We can imagine these
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bits breaking down as follows:
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```text
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10 . 1 . 2 . ? 0
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00001010 00000001 00000010 | XXXX | 0000
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parent network | netnum | host
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```
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Four of the eight bits that were originally the "host number" are now being
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repurposed as the subnet number. The network prefix no longer falls on an
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exact octet boundary, so in effect we are now splitting the last decimal number
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in the IP address into two parts, using half of it to represent the subnet
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number and the other half to represent the host number.
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The `netnum` argument then decides what number value to encode into those
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four new subnet bits. In our current example we passed `15`, which is
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represented in binary as `1111`, allowing us to fill in the `XXXX` segment
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in the above:
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```text
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10 . 1 . 2 . 15 0
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00001010 00000001 00000010 | 1111 | 0000
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parent network | netnum | host
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```
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To convert this back into normal decimal notation we need to recombine the
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two portions of the final octet. Converting `11110000` from binary to decimal
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gives 240, which can then be combined with our new prefix length of 28 to
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produce the result `10.1.2.240/28`. Again we can pass this prefix string to
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`ipcalc` to visualize it:
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```shell-session
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$ ipcalc 10.1.2.240/28
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Address: 10.1.2.240 00001010.00000001.00000010.1111 0000
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Netmask: 255.255.255.240 = 28 11111111.11111111.11111111.1111 0000
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Wildcard: 0.0.0.15 00000000.00000000.00000000.0000 1111
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=>
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Network: 10.1.2.240/28 00001010.00000001.00000010.1111 0000
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HostMin: 10.1.2.241 00001010.00000001.00000010.1111 0001
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HostMax: 10.1.2.254 00001010.00000001.00000010.1111 1110
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Broadcast: 10.1.2.255 00001010.00000001.00000010.1111 1111
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Hosts/Net: 14 Class A, Private Internet
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```
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The new subnet has four bits available for host numbering, which means
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that there are 14 host addresses available for assignment once we subtract
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the network's own address and the broadcast address. You can thus use
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[`cidrhost`](/nomad/docs/reference/hcl2/functions/ipnet/cidrhost) function to calculate those host addresses by
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providing it a value between 1 and 14:
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```shell-session
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> cidrhost("10.1.2.240/28", 1)
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10.1.2.241
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> cidrhost("10.1.2.240/28", 14)
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10.1.2.254
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```
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For more information on CIDR notation and subnetting, see
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[Classless Inter-domain Routing](https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing).
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## Related Functions
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- [`cidrhost`](/nomad/docs/reference/hcl2/functions/ipnet/cidrhost) calculates the IP address for a single host
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within a given network address prefix.
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- [`cidrnetmask`](/nomad/docs/reference/hcl2/functions/ipnet/cidrnetmask) converts an IPv4 network prefix in CIDR
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notation into netmask notation.
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- [`cidrsubnets`](/nomad/docs/reference/hcl2/functions/ipnet/cidrsubnets) can allocate multiple consecutive
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addresses under a prefix at once, numbering them automatically.
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