How is it that missing 0s are automatically added in IP addresses? (`ping 10.5` equivalent to `ping 10.0.0.5`)

Question

I accidentally typed

ssh 10.0.05

instead of

ssh 10.0.0.5

and was very surprised that it worked. I also tried 10.005 and 10.5 and those also expanded automatically to 10.0.0.5. I also tried 192.168.1 and that expanded to 192.168.0.1. All of this also worked with ping rather than ssh, so I suspect it would work for many other commands that connect to an arbitrary user-supplied host.

Why does this work? Is this behavior documented somewhere? Is this behavior part of POSIX or something? Or is it just some weird implementation? (Using Ubuntu 13.10 for what it's worth.)

Answer 1

Quoting from man 3 inet_aton:

   a.b.c.d   Each of the four numeric parts specifies a byte of the
             address; the bytes are assigned in left-to-right order to
             produce the binary address.

   a.b.c     Parts a and b specify the first two bytes of the binary
             address.  Part c is interpreted as a 16-bit value that
             defines the rightmost two bytes of the binary address.
             This notation is suitable for specifying (outmoded) Class B
             network addresses.

   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.

   a         The value a is interpreted as a 32-bit value that is stored
             directly into the binary address without any byte
             rearrangement.

   In all of the above forms, components of the dotted address can be
   specified in decimal, octal (with a leading 0), or hexadecimal, with
   a leading 0X).  Addresses in any of these forms are collectively
   termed IPV4 numbers-and-dots notation.  The form that uses exactly
   four decimal numbers is referred to as IPv4 dotted-decimal notation
   (or sometimes: IPv4 dotted-quad notation).

---

For fun, try this:

$ nslookup unix.stackexchange.com
Non-authoritative answer:
Name:   unix.stackexchange.com
Address: 198.252.206.140

$ echo $(( (198 << 24) | (252 << 16) | (206 << 8) | 140 ))
3338456716

$ ping 3338456716         # What?  What did we ping just now?
PING stackoverflow.com (198.252.206.140): 48 data bytes
64 bytes from 198.252.206.140: icmp_seq=0 ttl=52 time=75.320 ms
64 bytes from 198.252.206.140: icmp_seq=1 ttl=52 time=76.966 ms
64 bytes from 198.252.206.140: icmp_seq=2 ttl=52 time=75.474 ms

Answer 2

Adding to @devnull's fine answer, IPv4 addresses can be represented in the following ways.

Example

This domain name, google.com, can be represented in the following ways:

• 74.125.226.4  (dotted decimal)
• 1249763844  (flat decimal)
• 0112.0175.0342.0004  (dotted octal)
• 011237361004  (flat octal)
• 0x4A.0x7D.0xE2.0x04  (dotted hex)
• 0x4A7DE204  (flat hex)
• 74.0175.0xe2.4  (ಠ_ಠ)

Source: Why does pinging 192.168.072 (only 2 dots) return a response from 192.168.0.58?.