= Subnetter
Brent Saner <bts@square-r00t.net>
Last rendered {localdatetime}
:doctype: book
:docinfo: shared
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:toc: preamble
:toc2: left
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:source-highlighter: rouge
:docinfo: shared
:rfc: https://datatracker.ietf.org/doc/html/rfc
:arin_r: https://www.arin.net/participate/policy/nrpm/

[id="wat"]
== What is it?
A tool to assist in design of segregate/segment/split/subnet networks.

[id="out"]
== Output

* `PTP` refers to "Peer-to-Peer" (e.g. {rfc}3021[RFC 3021^]).
* `6rd` refers to "IPv6 Rapid Deployment", a derivation of 6to4 ({rfc}5569[RFC 5569^], {rfc}5969[RFC 5969^]).
* `LIR` refers to "Local Internet Registry" ({arin_r}#2-4-local-internet-registry-lir[ARIN^]).
* `RIR` refers to "Regional Internet Registry" ({arin_r}#2-2-regional-internet-registry-rir[ARIN^]).
* `Unicast` refers to "Global Unicast" ({rfc}1122[RFC 1122^], {rfc}4291#section-2.5.4[RFC 4291 § 2.5.4^], {rfc}4632[RFC 4632^]).
** For IPv6 addresses, it will be `true` for ULA (_Unique Local Addresses_) ({rfc}4193[RFC 4193^]) also.
** For IPv4 addresses, it will be `true` if the address is routable by external hosts (a unicast address), including private IP addresses ({rfc}1918[RFC 1918^]).
* `ILM` refers to "Interface-Local Multicast" ({rfc}4291#section-2.7[RFC 4291 § 2.7^], {rfc}7346[RFC 7346^]).
** It will always be `false` for IPv4 addresses.
* `LLM` refers to "Link-Local Multicast" ({rfc}4291#section-2.7[RFC 4291 § 2.7^], {rfc}7346[RFC 7346^]).
** For IPv4 addresses, it will be `true` if it is in the `224.0.0.0/4` range ({rfc}5735[RFC 5735^]).
* `LLU` refers to "Link-Local Unicast" ({rfc}4291#section-2.7[RFC 4291 § 2.7^], {rfc}7346[RFC 7346^]).
** For IPv4 addresses, it will be `true` if it is an APIPA (_Automatic Private IP Addressing_) address ({rfc}3927[RFC 3927^]) (in the `169.254.0.0/16` range).
* `First` and `Last` refer to the first and last "usable" ("host"/assignable) addresses in a subnet/network.
** Note that for IPv6, the first address (`x::`) in a subnet *may* or *may not* be assignable/"usable". If it is assigned to a device, that device *must* be a router for anycast. See {rfc}4291#section-2.6.1[RFC 4291 § 2.6.1^] for details. In the interest of convenience, `subnetter` will report this address as *not usable/addressable* in ranges for this reason as it is technically not a "host" address.
** Note that for IPv6, some subnetting calculators erroneously report the last address for /64's (e.g. `x:ffff:ffff:ffff:ffff/64`) as usable. They are actually reserved in strictly RFC-compliant networks for EUI-64 reasons (per {rfc}2526[RFC 2526^]). For this reason, *if and only if* a prefix is a /64 *exactly*, `subnetter` will use `x:ffff:ffff:ffff:fffe` as the last host address.
** There are additional restrictions for /64 subnets, but they fall earlier in the range. These are *not explicitly excluded* in the usable host range, nor are they excluded from the total host count.
* Private networks ({rfc}1918[RFC 1918^]), ULA prefixes ({rfc}4193[RFC 4193^]), and documentation prefixes ({rfc}3849[RFC 3849^], {rfc}5737[RFC 5737^], {rfc}9637[RFC 9637^]) are treated as "normal" networks (in that it is allowed to subnet them).
* Various other reserved IPv4 and IPv6 addresses/networks will print warnings with their corresponding RFC(s) (unless `-R`/`--allow-reserved` is specified) if they are specified as/included in the initial prefix/network.

[id="ref"]
== References
This program in general draws inspiration from `ipcalc` (http://jodies.de/ipcalc[0^], https://github.com/kjokjo/ipcalc[1^], https://gitlab.com/ipcalc/ipcalc[2^]) and http://www.routemeister.net/projects/sipcalc/[`sipcalc`^].

The `table` subcommand is inspired by `iptab` from https://metacpan.org/pod/Net::IP[Perl Net-IP^].

Additional notes for certain contexts are primarily taken from https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing[the Wikipedia article on _Classless Inter-Domain Routing_^] (as of _Jan 28, 2025_).