One real address block, six candidate addresses. Mark the ones that are inside the block, then check: every address is printed as 32 bits with the block's mask shown as a cut between the network half and the host half. That picture is the lesson — a prefix is not a number to memorise, it is a cut in the bit string.
Ten-round series 60-second run
Round 1 of 10. Mark every address that is inside the block, then check.
Score 0 / 0
Block
208.67.222.0/24
— Cisco OpenDNS
(AS36692) ·
US
Inside means inside the block's address range. The network and broadcast addresses are inside it — the reveal also shows which addresses a host can actually use.
Mask 11111111 11111111 11111111 00000000 (the
/24 cut) · range
208.67.222.0–208.67.222.255 ·
256 addresses, 254 usable by a host ·
first usable 208.67.222.1, last usable
208.67.222.254.
Every block is a real allocation: 1.1.1.0/24 is Cloudflare's, 8.8.8.0/24 is Google's, 128.141.0.0/16 is CERN's, 171.64.0.0/14 is Stanford's, 104.16.0.0/13 is Cloudflare's, 3.80.0.0/12 is Amazon's and 78.192.0.0/11 is Free's. The smaller blocks (/25 to /30) are sub-blocks of the holder's documented allocation, and the reveal says so.
Holder names come from this site's own hub data; the AS and country link to /asn/… and /country/… where those pages exist.
Best today in this browser — a series run and a 60-second run are each kept for the day. Nothing is sent to us.
One entry per UTC day, replaced only by a better run, and it never leaves this browser: there is no account and no score sent to us.
An IPv4 address is 32 bits. The number after the slash says how many of those bits,
counting from the left, belong to the network: 203.0.113.64/27 freezes the
first 27 and leaves the last 5 to the host. Everything else follows from that one
sentence.
10.1.0.0/22 runs from 10.1.0.0 to
10.1.3.255 — the third number moves. A /13 covers eight /16s, so
the second number moves. "The first three numbers are the same" is a /24 habit and it
is wrong for most prefixes.Anchors worth carrying around: /24 = 256 addresses, /25 = 128, /26 = 64, /27 = 32, /28 = 16, /29 = 8, /30 = 4 (2 usable); and the other way, /23 = 512, /22 = 1,024, /21 = 2,048, /20 = 4,096, /18 = 16,384, /16 = 65,536, /13 = 524,288. The ladder here walks from /24 through /25–/27 and /28–/30 to prefixes that cut inside an octet, because that is where the shortcut stops working.
The pool is chosen by the UTC day, hashed once, so every visitor and every instance sees
the same ladder on the same day — and a fresh one the next day, with no storage and no
account. A series is 10 rounds in order of difficulty, 6
points a round; the timed run walks the same day's pool of 24 rounds
against a 60-second clock. /subnet.htm?s=1,
?s=2, … are further ladders, each the same for everyone who asks for that
number, and ?m=timed is the timed mode.
The tools behind this mechanic: Connection card JSON API
Prefer countries? There is the daily IP puzzle. Prefer networks? There is the daily network puzzle, and Ping Detective infers a location from round-trip times. The network tools and the traceroute globe are the real measurements; this page is the arithmetic underneath them.
This page is the ten-round series for 2026-10-23. The same ladder is served to everyone on this UTC day.