Subnet cheat sheet

One page, US Letter, laid out to be pinned above a desk. It carries the same tables as this page and is generated from the same data, so the two cannot disagree.

Every prefix from /8 to /32

Usable host counts already have the network and broadcast addresses subtracted. The two exceptions are called out in the notes.

The two formulas

addresses  =  2 ^ (32 - prefix)
usable     =  2 ^ (32 - prefix) - 2      except /31 and /32

Going the other way, from a host requirement to a prefix: add two, round up to the next power of two, and that is your block size. See network, broadcast and usable hosts for the worked version.

Powers of two

The only table underneath all of this. Everything else is derived from it.

2^1  = 2        2^7  = 128        2^13 = 8,192
2^2  = 4        2^8  = 256        2^14 = 16,384
2^3  = 8        2^9  = 512        2^15 = 32,768
2^4  = 16       2^10 = 1,024      2^16 = 65,536
2^5  = 32       2^11 = 2,048      2^20 = 1,048,576
2^6  = 64       2^12 = 4,096      2^24 = 16,777,216

Mask octet values

A mask octet can only be one of these nine, because a mask is always ones followed by zeros. The block size column is 256 minus the value, which is the fastest route to a boundary.

Reserved and special-purpose ranges

Common IPv6 prefixes

Host counting stops being useful in IPv6, so the conventions are about structure instead.

A single /64 holds about 18 quintillion addresses, which is more than the whole IPv4 internet squared. Nobody subnets IPv6 to save addresses.