Some notes: These were tested on Linux using
ping. They may not apply to other operating systems.
If I know something is a Linux-specific quirk, I'll note that. Other tools may also have specific
restrictions on IPv4 formats, in particular, Rust tends to be pretty picky.
Localhost has an Entire /8 Block
As listed in RFC 6890, Section 2.2.2 Table 4, the entire 127.0.0.0/8 block is assigned to "Loopback". This means that all IP's between 127.0.0.1 and 127.255.255.254 are routed back to the local host.
Side note: 127.255.255.255 is a broadcast address, but where does that go?
Address Segments don't Have to be Decimal
Most of the time, IPv4 addresses are written as 4 decimal segments between 0 and 255. However, segments don't have to be written in decimal. Here are some other addresses that resolve to 127.0.0.1:
IPv4 Addresses don't need 4 Segments
Most of the time, IPv4 addresses don't need 4 segments, Linux appears to begin filling an IP address from the high-order bytes, then assumes the last segment fills from the lowest-order byte. Here's more ways to write 127.0.0.1
IPv4 Segments don't need to be < 255
Internally, the Linux kernel actually defines
this means that Linux will accept any number of 32 bit representations as an IPv4 address. Yet more ways
to write 127.0.0.1:
Linux also appears to interpret "0" as 127.0.0.1 for some reason? This one seems like a linux-specific behavior, neither Windows or MacOS replicate this.