Subnet Calculator and CIDR Notation Table

IPv4 address Mask or /Prefix Calculate FieldValue How to calculate manually (tips)

• Convert /prefix → mask: /24 means 24 ones then 8 zeros → 11111111.11111111.11111111.00000000 → 255.255.255.0
• Block size rule: In the first non-255 octet of the mask, do 256 − octet. Example: 255.255.255.192 ⇒ block size 256−192 = 64. Network ranges: ...0–63, 64–127, 128–191, 192–255.
• Network address: bitwise AND of IP and mask. Example: 192.168.10.130 & 255.255.255.192 = 192.168.10.128
• Broadcast address: all host bits set to 1. With /26 (64-block) starting at .128 ⇒ broadcast is .191
• Usable hosts: 2^(32−prefix) − 2 (skip network & broadcast) — except /31 (2 point-to-point) and /32 (host).
• First/Last usable: Network+1 and Broadcast−1 (for subnets where usable ≥2).