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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.
This commit is contained in:
Martin Atkins 2019-03-26 09:21:28 -07:00
parent a63c1af8e0
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10
website/docs/configuration/functions/cidrhost.html.md
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@ -25,7 +25,15 @@ cidrhost(prefix, hostnum)
`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
prefix.
prefix. For more details on how this function interprets CIDR prefixes and
populates host numbers, see the worked example for
[`cidrsubnet`](./cidrsubnet.html).
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.
This function accepts both IPv6 and IPv4 prefixes, and the result always uses
the same addressing scheme as the given prefix.

113
website/docs/configuration/functions/cidrsubnet.html.md
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@ -44,6 +44,119 @@ the same addressing scheme as the given prefix.
fd00:fd12:3456:7800:a200::/72
```
## 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:
```
10 . 1 . 2 . 0
00001010 00000001 00000010 | 00000000
network | host
```
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`](https://gitlab.com/ipcalc/ipcalc) is useful for
visualizing CIDR prefixes as binary numbers. We can confirm the conversion
above by providing the same prefix string to `ipcalc`:
```
$ ipcalc 10.1.2.0/24
Address: 10.1.2.0 00001010.00000001.00000010. 00000000
Netmask: 255.255.255.0 = 24 11111111.11111111.11111111. 00000000
Wildcard: 0.0.0.255 00000000.00000000.00000000. 11111111
=>
Network: 10.1.2.0/24 00001010.00000001.00000010. 00000000
HostMin: 10.1.2.1 00001010.00000001.00000010. 00000001
HostMax: 10.1.2.254 00001010.00000001.00000010. 11111110
Broadcast: 10.1.2.255 00001010.00000001.00000010. 11111111
Hosts/Net: 254 Class A, Private Internet
```
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`](./cidrhost.html) 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:
```
10 . 1 . 2 . ? 0
00001010 00000001 00000010 | XXXX | 0000
parent network | netnum | host
```
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:
```
10 . 1 . 2 . 15 0
00001010 00000001 00000010 | 1111 | 0000
parent network | netnum | host
```
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:
```
$ ipcalc 10.1.2.240/28
Address: 10.1.2.240 00001010.00000001.00000010.1111 0000
Netmask: 255.255.255.240 = 28 11111111.11111111.11111111.1111 0000
Wildcard: 0.0.0.15 00000000.00000000.00000000.0000 1111
=>
Network: 10.1.2.240/28 00001010.00000001.00000010.1111 0000
HostMin: 10.1.2.241 00001010.00000001.00000010.1111 0001
HostMax: 10.1.2.254 00001010.00000001.00000010.1111 1110
Broadcast: 10.1.2.255 00001010.00000001.00000010.1111 1111
Hosts/Net: 14 Class A, Private Internet
```
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`](./cidrhost.html) function to calculate those host addresses by
providing it a value between 1 and 14:
```
> cidrhost("10.1.2.240/28", 1)
10.1.2.241
> cidrhost("10.1.2.240/28", 14)
10.1.2.254
```
For more information on CIDR notation and subnetting, see
[Classless Inter-domain Routing](https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing).
## Related Functions
* [`cidrhost`](./cidrhost.html) calculates the IP address for a single host