[j1elo]

> It does not process Class A/B notation, or hex or octal notation.

I got to find that notation useful once, to make a shorter one-liner... without even knowing that there were different classes of IPv4 address, and that I was looking at one of them.

It's a tiny function that gives me the IP address of my machine in the LAN, for either Linux and Mac:

  # Get main local IP address from the default external route (Internet gateway)
  iplan() {
      # Note: "1" is shorthand for "1.0.0.0"
      case "$OSTYPE" in
          linux*) ip -4 -oneline route get 1 | grep -Po 'src \K([\d.]+)' ;;
          darwin*) ipconfig getifaddr "$(route -n get 1 | sed -n 's/.*interface: //p')" ;;
      esac
  }

(sorry to people reading on small screens)

Full disclosure, I got the "1 is shorthand for 1.0.0.0" from here (which didn't get into explaining why it is a shorthand): https://stackoverflow.com/a/25851186

[dave_universetf]

Oh no, that's another shorthand that's different from all the others. A single number should be interpreted as a big-endian uint32, and so "1" should be "0.0.0.1". However, I can confirm that `ip` interprets it as "1.0.0.0", even though you should have to write "1.0" for that.

Ugh.

[j1elo]

Well I did think of that, it technically is not a Class-A because it should have 2 parts. My conclusion was that maybe what happens is that "1", while incorrect, is flexibly parsed as "1.0" and thus it would become "1.0.0.0". But you're right that, given the uint32 representation does exist, the most correct thing to do seems to interpret it as "0.0.0.1"...

unless an exception to the rule exists somewhere, and 'ip' is actually doing it right!

[gravitas]

I was curious as well, turns out they're using a non-standard parsing with a comment in the function explaining why:

    /* This uses a non-standard parsing (ie not inet_aton, or inet_pton)
     * because of legacy choice to parse 10.8 as 10.8.0.0 not 10.0.0.8
     */

src: https://git.kernel.org/pub/scm/network/iproute2/iproute2.git...

(the entry point to start tracing down to the above inner function is right around here: https://git.kernel.org/pub/scm/network/iproute2/iproute2.git... )

[j1elo]

Conclusion: explicit is better than implicit, and what's more, in this case the implicit alternative was depending on a non-standard choice made in the specific tool for obscure, legacy reasons.

[gravitas]

This fits almost any situation (explicit vs. implicit) and I'm a big fan - when mentoring I tend to say "yes that was the default when you looked today, how do you know it won't change tomorrow? If you want specific behaviour, be explicit don't trust defaults." (more or less, depends on subject - commandline switches to code loops, same advice)

[FreshFries]

macOS here:

% ping 1 PING 1 (0.0.0.1): 56 data bytes

% ping 1.0 PING 1.0 (1.0.0.0): 56 data bytes

[anderskaseorg]

Did you really gain anything here, given that the omission of those 12 characters required a 38 character comment to explain what’s going on?

[j1elo]

Absolutely no :)

What I wanted to express here (and did badly) is that crossing paths with this arcane Class-A style IP address is something so strange nowadays... in my case in more than 10 years professionally working as a developer, I had seen it exactly once and even then, didn't recognize it for what it was.

The code snippet was just an extra curiosity in case anyone found it useful.