Subnetting with Fingers

I just memorize these sequences, ungainly, but works.

Decimal masks - 128, 192, 224, 240, 248, 252, 254, 255

Wildcard masks - 127, 63, 31, 15, 7, 3, 1, 0

Subnet Sizes (going up) - 256, 512, 1024, 2048

Subnet Sizes (going down) - 128, 64, 32, 16, 8, 4, 2, 1

Subnet Examples

/24 is 256 IPs. Most gear complains if you use .0 or .256, so we say 254 usable hosts.

/30 is the 1990s way of addressing a point-to-point link, which wastes two IPs.

/31 is exactly two IPs. This is the best subnet for point-to-point links.

/32 is a single address. We call these host routes. 8.8.8.8/32 is Google’s DNS.