Maybe it means a non-reserved IP? 127.0.0.1, is often used as localhost for example, but all of 127.0.0.0–127.255.255.255 is reserved. There are several reserved blocks like this. If you include reserved addresses, every 32 bit pattern is a valid ipv4 address at least in theory. I hear network equipment in the real world is entirely another beast. I remember some ISPs did not work cloudflare's 1.1.1.1 DNS presumably because they were using the address for something else.

My personal favorite is assuming that /31 IP subnets (255.255.255.254) are invalid (RFC 3021), but also, to quote wikipedia:

> IPv4 addresses may be represented in any notation expressing a 32-bit integer value. They are most often written in dot-decimal notation, which consists of four octets of the address expressed individually in decimal numbers and separated by periods.

> For example, the quad-dotted IP address 192.0.2.235 represents the 32-bit decimal number 3221226219, which in hexadecimal format is 0xC00002EB. This may also be expressed in dotted hex format as 0xC0.0x00.0x02.0xEB, or with octal byte values as 0300.0000.0002.0353.

I think that is because not every IPv4 adress has the format x.x.x.x There are short forms (i.e. 1.1 that gets expanded to 1.0.0.1, though not sure if that's in the original RFC) or decimal forms. And then there's IPv6, which again has its rules.