The one number that matters: block size
An IPv4 address has 32 bits. A prefix like /26 says how many of those bits are the network part. The rest are host bits:
/24→ 8 host bits → 2⁸ = 256 addresses/26→ 6 host bits → 2⁶ = 64 addresses/29→ 3 host bits → 2³ = 8 addresses
That count is the block size. Networks always start on a multiple of the block size.
Network and broadcast in your head
Take 192.168.10.77/26. The block size is 64, so networks in the last octet start at 0, 64, 128, 192. 77 falls in the block that starts at 64:
- Network:
192.168.10.64 - Broadcast:
192.168.10.127(next block minus one) - Usable hosts:
.65to.126→ 62 hosts (64 − 2)
Quick table
/24— mask 255.255.255.0 — 256 addresses, 254 usable/25— mask 255.255.255.128 — 128 addresses, 126 usable/26— mask 255.255.255.192 — 64 addresses, 62 usable/27— mask 255.255.255.224 — 32 addresses, 30 usable/28— mask 255.255.255.240 — 16 addresses, 14 usable/29— mask 255.255.255.248 — 8 addresses, 6 usable/30— mask 255.255.255.252 — 4 addresses, 2 usable
/31 (point-to-point links, RFC 3021) and /32 (a single host) are the special cases where the "minus two" rule does not apply.
Check your answer
Use the subnet calculator on this site — it does exactly this arithmetic with bitwise operations.