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Comment by nerdwaller | Hacker News Reader

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

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

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

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