DAILY PUZZLE · NEW EVERY DAY
SUBNETLE
One subnet a day. Work out the network, broadcast, first usable address and number of usable hosts in six tries.
Loading puzzle…
| # | Network | Broadcast | First usable | Hosts |
|---|
HOW TO PLAY
- correct
- close, the answer is higher
- far off, the answer is higher
- close, the answer is lower
- far off, the answer is lower
- not a valid address or number
"Close" means within one block of the answer for addresses, or within a factor of two for the host count. Everyone gets the same puzzle each day, based on your local date. Your streak is saved in this browser only.
HOW TO SOLVE A SUBNET
Take 172.18.77.140/21. A /21 leaves 32 − 21 = 11 host bits, so each block holds 211 = 2,048 addresses. The prefix ends in the third octet, where the block size is 2(24 − 21) = 8.
- Network: round the third octet down to a multiple of 8: 77 becomes 72, and the fourth octet becomes 0. Network =
172.18.72.0. - Broadcast: the next block starts at 72 + 8 = 80, so the broadcast is one address earlier:
172.18.79.255. - First usable: network + 1 =
172.18.72.1. - Usable hosts: 2,048 − 2 = 2,046 (the network and broadcast addresses can't be assigned).
Special cases: a /31 has 2 usable addresses on point-to-point links (RFC 3021), and a /32 is a single host.