[Jtsummers]

127.1 is the same as 127.0.0.1. 0 bytes are inserted based on the following:

  x     -> 0.0.0.x
  x.y   -> x.0.0.y
  x.y.z -> x.y.0.z

[bryanlarsen]

But sometimes 127.1 means 127.1.0.0/16. For example in the output of `netstat -rn` on MacOS.

[Jtsummers]

Good to know.

I was also coming back in to edit my comment and write that my x -> 0.0.0.x was technically wrong and the same for the others. If you stick to only single-byte values then what I wrote was correct but if the values span multiple bytes they fill in the missing 0s appropriately, as in:

  256   -> 0.0.1.0
  1.256 -> 1.0.1.0

[elcritch]

Similar rules apply to ipv6 addresses as well.

[jandrese]

IPv6 at least sensibly has a different delimiter for when you are eliding zeros.

[moefh]

It's not really about eliding zeros, though. It's kind of a cursed thing:

  x       -> x3.x2.x1.x0
  x.y     ->  x.y2.y1.y0
  x.y.z   ->  x. y.z1.z0
  x.y.z.w ->  x. y. z. w

Where x0 is the 8 least significant bits of x, x1 is the next 8 higher bits, and so on.

The "zeros" happen when the last number is smaller than 256 (or 65536, etc.), but it doesn't have to be. For example 10.258 is a valid way to write an IPv4 address, it's the same as 10.0.1.2.

[ice3]

Here's a cursed way of writing an ip address: 0xA.024.7720 where 0xA is hex, 024 is octal and last 2 octets are written as a decimal number