github.com/muratcelep/terraform@v1.1.0-beta2-not-internal-4/website/docs/language/functions/cidrsubnet.html.md (about) 1 --- 2 layout: "language" 3 page_title: "cidrsubnet - Functions - Configuration Language" 4 sidebar_current: "docs-funcs-ipnet-cidrsubnet" 5 description: |- 6 The cidrsubnet function calculates a subnet address within a given IP network 7 address prefix. 8 --- 9 10 # `cidrsubnet` Function 11 12 `cidrsubnet` calculates a subnet address within given IP network address prefix. 13 14 ```hcl 15 cidrsubnet(prefix, newbits, netnum) 16 ``` 17 18 `prefix` must be given in CIDR notation, as defined in 19 [RFC 4632 section 3.1](https://tools.ietf.org/html/rfc4632#section-3.1). 20 21 `newbits` is the number of additional bits with which to extend the prefix. 22 For example, if given a prefix ending in `/16` and a `newbits` value of 23 `4`, the resulting subnet address will have length `/20`. 24 25 `netnum` is a whole number that can be represented as a binary integer with 26 no more than `newbits` binary digits, which will be used to populate the 27 additional bits added to the prefix. 28 29 This function accepts both IPv6 and IPv4 prefixes, and the result always uses 30 the same addressing scheme as the given prefix. 31 32 Unlike the related function [`cidrsubnets`](./cidrsubnets.html), `cidrsubnet` 33 allows you to give a specific network number to use. `cidrsubnets` can allocate 34 multiple network addresses at once, but numbers them automatically starting 35 with zero. 36 37 -> **Note:** As a historical accident, this function interprets IPv4 address 38 octets that have leading zeros as decimal numbers, which is contrary to some 39 other systems which interpret them as octal. We have preserved this behavior 40 for backward compatibility, but recommend against relying on this behavior. 41 42 ## Examples 43 44 ``` 45 > cidrsubnet("172.16.0.0/12", 4, 2) 46 172.18.0.0/16 47 > cidrsubnet("10.1.2.0/24", 4, 15) 48 10.1.2.240/28 49 > cidrsubnet("fd00:fd12:3456:7890::/56", 16, 162) 50 fd00:fd12:3456:7800:a200::/72 51 ``` 52 53 ## Netmasks and Subnets 54 55 Using `cidrsubnet` requires familiarity with some network addressing concepts. 56 57 The most important idea is that an IP address (whether IPv4 or IPv6) is 58 fundamentally constructed from binary digits, even though we conventionally 59 represent it as either four decimal octets (for IPv4) or a sequence of 16-bit 60 hexadecimal numbers (for IPv6). 61 62 Taking our example above of `cidrsubnet("10.1.2.0/24", 4, 15)`, the function 63 will first convert the given IP address string into an equivalent binary 64 representation: 65 66 ``` 67 10 . 1 . 2 . 0 68 00001010 00000001 00000010 | 00000000 69 network | host 70 ``` 71 72 The `/24` at the end of the prefix string specifies that the first 24 73 bits -- or, the first three octets -- of the address identify the network 74 while the remaining bits (32 - 24 = 8 bits in this case) identify hosts 75 within the network. 76 77 The CLI tool [`ipcalc`](https://gitlab.com/ipcalc/ipcalc) is useful for 78 visualizing CIDR prefixes as binary numbers. We can confirm the conversion 79 above by providing the same prefix string to `ipcalc`: 80 81 ``` 82 $ ipcalc 10.1.2.0/24 83 Address: 10.1.2.0 00001010.00000001.00000010. 00000000 84 Netmask: 255.255.255.0 = 24 11111111.11111111.11111111. 00000000 85 Wildcard: 0.0.0.255 00000000.00000000.00000000. 11111111 86 => 87 Network: 10.1.2.0/24 00001010.00000001.00000010. 00000000 88 HostMin: 10.1.2.1 00001010.00000001.00000010. 00000001 89 HostMax: 10.1.2.254 00001010.00000001.00000010. 11111110 90 Broadcast: 10.1.2.255 00001010.00000001.00000010. 11111111 91 Hosts/Net: 254 Class A, Private Internet 92 ``` 93 94 This gives us some additional information but also confirms (using a slightly 95 different notation) the conversion from decimal to binary and shows the range 96 of possible host addresses in this network. 97 98 While [`cidrhost`](./cidrhost.html) allows calculating single host IP addresses, 99 `cidrsubnet` on the other hand creates a new network prefix _within_ the given 100 network prefix. In other words, it creates a subnet. 101 102 When we call `cidrsubnet` we also pass two additional arguments: `newbits` and 103 `netnum`. `newbits` decides how much longer the resulting prefix will be in 104 bits; in our example here we specified `4`, which means that the resulting 105 subnet will have a prefix length of 24 + 4 = 28 bits. We can imagine these 106 bits breaking down as follows: 107 108 ``` 109 10 . 1 . 2 . ? 0 110 00001010 00000001 00000010 | XXXX | 0000 111 parent network | netnum | host 112 ``` 113 114 Four of the eight bits that were originally the "host number" are now being 115 repurposed as the subnet number. The network prefix no longer falls on an 116 exact octet boundary, so in effect we are now splitting the last decimal number 117 in the IP address into two parts, using half of it to represent the subnet 118 number and the other half to represent the host number. 119 120 The `netnum` argument then decides what number value to encode into those 121 four new subnet bits. In our current example we passed `15`, which is 122 represented in binary as `1111`, allowing us to fill in the `XXXX` segment 123 in the above: 124 125 ``` 126 10 . 1 . 2 . 15 0 127 00001010 00000001 00000010 | 1111 | 0000 128 parent network | netnum | host 129 ``` 130 131 To convert this back into normal decimal notation we need to recombine the 132 two portions of the final octet. Converting `11110000` from binary to decimal 133 gives 240, which can then be combined with our new prefix length of 28 to 134 produce the result `10.1.2.240/28`. Again we can pass this prefix string to 135 `ipcalc` to visualize it: 136 137 ``` 138 $ ipcalc 10.1.2.240/28 139 Address: 10.1.2.240 00001010.00000001.00000010.1111 0000 140 Netmask: 255.255.255.240 = 28 11111111.11111111.11111111.1111 0000 141 Wildcard: 0.0.0.15 00000000.00000000.00000000.0000 1111 142 => 143 Network: 10.1.2.240/28 00001010.00000001.00000010.1111 0000 144 HostMin: 10.1.2.241 00001010.00000001.00000010.1111 0001 145 HostMax: 10.1.2.254 00001010.00000001.00000010.1111 1110 146 Broadcast: 10.1.2.255 00001010.00000001.00000010.1111 1111 147 Hosts/Net: 14 Class A, Private Internet 148 ``` 149 150 The new subnet has four bits available for host numbering, which means 151 that there are 14 host addresses available for assignment once we subtract 152 the network's own address and the broadcast address. You can thus use 153 [`cidrhost`](./cidrhost.html) function to calculate those host addresses by 154 providing it a value between 1 and 14: 155 156 ``` 157 > cidrhost("10.1.2.240/28", 1) 158 10.1.2.241 159 > cidrhost("10.1.2.240/28", 14) 160 10.1.2.254 161 ``` 162 163 For more information on CIDR notation and subnetting, see 164 [Classless Inter-domain Routing](https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing). 165 166 ## Related Functions 167 168 * [`cidrhost`](./cidrhost.html) calculates the IP address for a single host 169 within a given network address prefix. 170 * [`cidrnetmask`](./cidrnetmask.html) converts an IPv4 network prefix in CIDR 171 notation into netmask notation. 172 * [`cidrsubnets`](./cidrsubnets.html) can allocate multiple consecutive 173 addresses under a prefix at once, numbering them automatically.