Quiz Entry - updated: 2026.07.14
What changes when you subnet a /16 prefix (e.g. 172.16.0.0/16) compared with a /24?
Borrowing bits from a /16 borrows into the 3rd octet first; e.g. /17 = 2 subnets/32766 hosts, /24 = 256 subnets/254 hosts, /26 = 1024 subnets/62 hosts.
A /16 starts with 16 host bits (3rd and 4th octets), so you have far more bits to borrow than with a /24.
| Prefix | Mask | # Subnets | # Hosts each |
|---|---|---|---|
| /17 | 255.255.128.0 | 2 | 32,766 |
| /18 | 255.255.192.0 | 4 | 16,382 |
| /19 | 255.255.224.0 | 8 | 8,190 |
| /20 | 255.255.240.0 | 16 | 4,094 |
| /22 | 255.255.252.0 | 64 | 1,022 |
| /24 | 255.255.255.0 | 256 | 254 |
| /26 | 255.255.255.192 | 1,024 | 62 |
Why it matters: the last two host bits can never be borrowed (a subnet needs at least 2 host bits for a usable network + broadcast), so a /16 can be subnetted no further than /30.
Example — needing ≥ 100 subnets from 172.16.0.0/16: borrow 7 bits (2^7 = 128 ≥ 100) → /23.
Go deeper:
Visual Subnet Calculator (davidc.net) — start with a /16 and watch how borrowing bits reaches into the 3rd octet first.
Classless Inter-Domain Routing (Wikipedia) — prefix arithmetic across any starting length.