Subnet Cheat Sheet

The Core Pattern

Mental Shortcut

Broadcast & Host Range — Always the Same Trick

The 5-Question Flow — Same Every Time

1. Netmask  → how many octets are locked? (slash ÷ 8)
2. Host bits  → 32 minus the slash number
3. Hosts  → 2^{(host bits)} − 2
4. Network addr  → given address, last free octet = 0
5. Broadcast  → last free octet = 255
6. Range  → .1 to .254 across the free octets

Split Octets — What Your Mind Should Do

Octet bit values, left to right (always this order):

Example above shows /28 — 4 bits stolen (28 − 24 = 4). Yellow = locked, grey = free.

The mental path — step by step for any split CIDR:

• Remainder slash ÷ 8 → quotient = full locked octets, remainder = bits stolen /25: 25÷8 = 3 R1 → 1 bit stolen
• Stolen value read the bit value from the table above (start at 128, count right) 1 bit → 128  |  2 bits → 128+64=192  |  3 bits → 224
• Netmask last octet = sum of stolen bit values; full octets before it = 255 /25 → 255.255.255.128
• Block size 256 − last-octet netmask  (or: value of the last stolen bit) 256 − 128 = 128  →  subnets start at .0 and .128
• Network addr round the 4th octet of your address down to the nearest multiple of block size 192.168.1.50/25 → 50 rounds to 0 → network = 192.168.1.0
• Broadcast network 4th-octet + block − 1 0 + 128 − 1 = 127 → broadcast = 192.168.1.127
• Host range network+1 to broadcast−1 192.168.1.1 → 192.168.1.126

Split CIDR Quick-Reference

Worked Example — 192.168.10.0/24

Practice Problems