Subnetter

Table of Contents

• 1. What is it?
• 2. Output
• 3. References

1. What is it?

A tool to assist in design of segregate/segment/split/subnet networks.

2. Output

• Unicast refers to "Global Unicast" (RFC 1122, RFC 4291 § 2.5.4, RFC 4632).

  • For IPv6 addresses, it will be true for ULA (Unique Local Addresses) (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).

• ILM refers to "Interface-Local Multicast" (RFC 4291 § 2.7, RFC 7346).

  • It will always be false for IPv4 addresses.

• LLM refers to "Link-Local Multicast" (RFC 4291 § 2.7, RFC 7346).

  • For IPv4 addresses, it will be true if it is in the 224.0.0.0/4 range (RFC 5735).

• LLU refers to "Link-Local Unicast" (RFC 4291 § 2.7, RFC 7346).

  • For IPv4 addresses, it will be true if it is an APIPA (Automatic Private IP Addressing) address (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 § 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). 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), ULA prefixes (RFC 4193), and documentation prefixes (RFC 3849, RFC 5737, 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.

3. References

This program in general draws inspiration from ipcalc (0, 1, 2) and sipcalc.

The table subcommand is inspired by iptab from Perl Net-IP.

Additional notes for certain contexts are primarily taken from the Wikipedia article on Classless Inter-Domain Routing (as of Jan 28, 2025).