App: CIDR Trainer
Developer: Network Mom LLC
Genres: Developer Tools, Education

CIDR Trainer trains users to perform network subnet calculations.  It uses directed training drills while providing documentation to instruct users to perform each step of network subnetting calculations.

CIDR Trainer includes IPv4 and IPv6 CIDR calculators.

Version 1.1 includes the following categories of drills:

1-digit hexadecimal and binary drills
2-digit hexadecimal and binary drills
IPv4 terminology drills
IPv4 prefix-length drills
IPv4 CIDR drills
IPv6 Address Shortening and Lengthening Drills
IPv6 Terminology Drills
IPv6 CIDR Drills
IPv4 and IPv6 CIDR Calculators

You may try each training drill for free 20 times.  After that you may try the other drills or purchase a permanent unlock of all training drills for a low price (see in-app purchases for the "forever full-unlock" price in your region).  One purchase unlocks all versions of the app (iOS, iPadOS, and MacOS).  CIDR Trainer uses iCloud to sync drill progress between your devices.

Each training drill has a "help" button in the top-right which provides related documentation

Full list of drills: (you may try them in any order)

1-digit hex -> decimal
1-digit decimal -> hex
1-digit hex -> binary
Binary -> 1-digit hex
4-digit binary -> decimal
Decimal -> 4-digit binary
1-bit AND

2-digit hexadecimal -> decimal (easy multiples of 16)
Decimal -> 2-digit hexadecimal (easy multiples of 16)
2-digit hexadecimal -> binary
binary -> 2-digit hexadecimal
2-digit hexadecimal -> decimal
Decimal -> 2-digit hexadecimal
8-digit binary -> decimal
Decimal -> 8-digit binary
8-bit AND

Valid IPv4 address recognition
IPv4 address types

IPv4 prefix-length to binary netmask
IPv4 binary netmask to prefix-length
IPv4 prefix-length to hexadecimal netmask
IPv4 hexadecimal netmask to prefix-length
IPv4 prefix-length to dotted-decimal netmask
IPv4 dotted-decimal netmask to prefix-length

IPv4 CIDR Drills:
  Number of IPs in a CIDR
  Number of usable unicast IPs in a CIDR
  CIDR to network address in binary
  CIDR to network address in decimal
  Well-formed CIDR to broadcast address
  Well-formed CIDR to first usable IP address
  Well-formed CIDR to last usable IP address

IPv6 Address Shortening and Lengthening Drills:
  IPv6 Hextet Shortening
  IPv6 Hextet Lengthening
  IPv6 Eight Hextet Shortening
  IPv6 Eight Hextet Lengthening
  IPv6 Double Colon Shortening
  IPv6 Double Colon Lengthening
  IPv6 Address Shortening
  IPv6 Address Lengthening

IPv6 Address Types
Valid IPv6 Sextets
Valid IPv6 Addresses

IPv6 CIDR (prefix / 16) drill
IPv6 CIDR (prefix / 4) drill
IPv6 CIDR (any prefix) drill

IPv4 CIDR Calculator
IPv6 CIDR Calculator

Release notes:
Fixed a bug that could cause a crash in IPv4 address type drill
Fixed an error in the IPv6 address lengthening drill for :: and ::1 cases
Several cosmetic fixes
Changed iOS support to start at 15.0 instead of 15.2