[tgsovlerkhgsel]

A netblock. A "/n" means "a block of IP addresses where the first n bits are the same".

This comes from the CIDR notation 192.168.1.0/24 indicating the netblock from 192.168.1.0 - 192.168.1.255.

So the number after the slash corresponds to a network size. The smaller the number, the more bits are variable, i.e. the bigger the netblock.

An IPv4 /24 has 32 (address length) - 24 = 8 bits that are variable, i.e. 2^8 addresses. A /18 would have 32-18 = 14 variable bits, i.e. 2^14 addresses.

In IPv6, the address length is 128. A /64 (which happens to be the smallest routable network size in IPv6) is 2^64 addresses - but because it's easy to get large IPv6 netblocks (typical ISP allocations are /64, /56, and you can often get a /48 with relative ease), for abuse blocking purposes, you generally treat the entire /64 or even /56 as one entity (i.e. you ignore the rightmost 64 or 72 bits).

A /56 is 2^8 separate /64s. A /48 is 2^16 separate /64s.

[proto-n]

That was very informative for me, thanks!