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Comment by dieulot | Hacker News Reader

The most useful case for this shortcut is 127.1 -> 127.0.0.1

Don't try that in the wild, most sw out there would ignore spec and use some arbitrary regex to validate IP format.

i.e python:

    octets = ip_str.split('.')
    if len(octets) != 4:
        raise AddressValueError("Expected 4 octets in %r" % ip_str)

What spec says that 127.1 and 127.0.0.1 are equivalent?

I don’t actually think it’s in a spec formally but is in a common c lib[0].

> a.b

> Part a specifies the first byte of the binary address. Part b is interpreted as a 24-bit value that defines the rightmost three bytes of the binary address. This notation is suitable for specifying (outmoded) Class C network addresses.

[0]: https://linux.die.net/man/3/inet_aton

Thanks. I just came across the man page myself while I was writing this tiny program.

  $ cat 127.1.c
  #include <stdio.h>
  #include <arpa/inet.h>
   
  int main(int argc, char *argv[])
  {
      struct in_addr addr;
   
      if (inet_aton(argv[1], &addr))
          printf("%08x\n", addr.s_addr);
   
      return 0;
  }
  $ make 127.1 CFLAGS=-Wall
  cc -Wall     127.1.c   -o 127.1
  $ ./127.1 1.1
  01000001
  $ ./127.1 127.1
  0100007f

The POSIX spec (IEEE 1003.1) says the same thing for inet_addr(), so it does occur in an actual spec.

You are right, I faithfully assumed it's a spec without checking. Thanks.

0, which is a shorthand for 0.0.0.0 is likely the most code-golf-y way to write localhost, as many [EDIT: Linux] systems alias 0.0.0.0 to 127.0.0.1:

  $ ping 0
  PING 0 (127.0.0.1) 56(84) bytes of data.
  64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.032 ms

Of course, don't expect this to work universally. A lot of software will try to be clever with input validation, and fail.

Stays unaliased on macOS:

My-MacBook-Pro:bottle mrkstu$ ping 0 PING 0 (0.0.0.0): 56 data bytes ping: sendto: No route to host

However ping to 127.1 works the same as localhost.

It's not fully true that 127.0.0.1 is the same as 0.0.0.0. For example, binding a webserver to 0.0.0.0 make it on the public network while 127.0.0.1 is strictly localhost.

0.0.0.0 is not localhost. It's "any address".

Yes, you're right.

What I was trying to say is - On Linux, INADDR_ANY (0.0.0.0) supplied to connect() or sendto() calls is treated as a synonym for INADDR_LOOPBACK (127.0.0.1) address.

Not so for bind() or course.

Where have you been all my life?

Sitting at 127.1, apparently.

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