Krill KitsKrill Kits// A swarm of small, sharp tools for letters, numbers, and units.
§ 01 / TOOL

Network Class Lookup.

STATUS READYSCHEME IPv4 CLASSFULSTORAGE NONE
> IPv4 ADDRESS
CLASSFUL LOOKUP

Result.

PRIVATE
  • CLASS
    C
  • DEFAULT MASK
    255.255.255.0
  • DEFAULT PREFIX
    /24
  • RANGE
    192.0.0.0 – 223.255.255.255
// FLAGS
  • PRIVATE
    192.168.0.0/16 — RFC 1918 private (Class C).

All classes.

REFERENCE
  • A
    0.0.0.0 – 127.255.255.255
    Large networks (16M hosts each). — /8 (255.0.0.0)
  • B
    128.0.0.0 – 191.255.255.255
    Mid-size networks (65K hosts each). — /16 (255.255.0.0)
  • C
    192.0.0.0 – 223.255.255.255
    Small networks (254 hosts each). — /24 (255.255.255.0)
  • D
    224.0.0.0 – 239.255.255.255
    Multicast group addresses.
  • E
    240.0.0.0 – 255.255.255.255
    Reserved for experimental / future use.
§ 02 / ABOUT

Classful addressing.

Before CIDR (1993), IPv4 addresses were divided into fixed-size classes based on the first octet. The class determined how much of the address was the network ID vs the host ID — A = first octet, B = first two, C = first three. Today CIDR replaced this with variable-length masks, but the class vocabulary is still everywhere.

// THE CLASSES

  • Class A — 0.0.0.0–127.255.255.255 — default mask /8 (255.0.0.0). Originally for very large networks.
  • Class B — 128.0.0.0–191.255.255.255 — /16 (255.255.0.0). Mid-size organizations.
  • Class C — 192.0.0.0–223.255.255.255 — /24 (255.255.255.0). Small networks.
  • Class D — 224.0.0.0–239.255.255.255 — multicast.
  • Class E — 240.0.0.0–255.255.255.255 — reserved/experimental.

// THE FLAGS THIS TOOL CHECKS

  • Private (RFC 1918) — 10/8, 172.16/12, 192.168/16
  • Loopback — 127/8 (always your own machine)
  • Link-local (APIPA) — 169.254/16 (DHCP failed)
  • CGNAT — 100.64/10 (carrier-grade NAT)
  • Test/Doc — 192.0.2/24, 198.51.100/24, 203.0.113/24
  • Multicast/Reserved/Broadcast — D, E, 255.255.255.255
§ 03 / FAQ

Class questions.

Is classful addressing still relevant?+
Mostly historical, but the vocabulary persists. Modern networks use CIDR (variable-length subnet masking), which mostly replaced classful design in the 1990s. But you’ll still hear "Class C network" as shorthand for a /24, and the default masks are common defaults in switch and router configs.
What’s the difference between public and private?+
Public IPs are globally routable on the internet. Private IPs (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are reserved by RFC 1918 for internal networks — they get rewritten to your public IP via NAT when traffic leaves the network. Public is what websites see; private is what your home router hands out.
What’s 100.64.0.0/10 (CGNAT)?+
Carrier-grade NAT — RFC 6598. Reserved for ISPs that ran out of public IPv4 to give every customer one. Multiple customers share an outward-facing IP behind the ISP’s NAT. Not strictly "private" in the RFC 1918 sense but functionally similar.
What about IPv6?+
IPv6 doesn’t use classes — the address space is huge enough that no rationing was needed. The IPv6 equivalents of "private" are link-local (fe80::/10) and unique local (fc00::/7). This tool handles IPv4 only.
§ 04 / TOOLS

Related calculators.

§ 05 / READING

Deeper dives.