Where are you seeing a double colon? He write "10.1", not "10::1".

IPv4 addresses, according to POSIX, can be written in 4 forms:

  A1.A2.A3.A4
  A1.A2.B
  A1.C
  D

where Ai is an 8 bit number, B is a 16 bit number, C is a 24 bit number, and D is a 32 bit number. "10.1" is A1.C form.

See the inet_addr man page if you are on Unix or a Unix-like system.

Great explanation thank you! The 24 bit and 16 bit variants are highly useable but they are often overlooked by regexps, biggest problem is web apps. Being able to use octal and hex is even less common.

I referred to the double colon as something I saw used with IPv6 to indicate a sequence of 0's.

I wasn't aware that inet_aton supported all these forms and the regex I provide won't parse them. It seems like inet_aton supports specifying the numbers in octal and hex too.