1.1.1.1: Fast, privacy-first consumer DNS service

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)

Where have you been all my life?

Sitting at 127.1, apparently.

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

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.

Tangentially related: https://fosdem.org/2018/schedule/event/email_address_quiz/

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

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

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

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".

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

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.

However ping to 127.1 works the same as localhost.