Why can I access a server by a partial IP address?

Question

In my network I have a server known by the IP address 10.0.0.15. By accident, I discovered that the command: ping 10.0.15 results in

64 bytes from 10.0.0.15: icmp_seq=1 ttl=64 time=9.09 ms

...so the correct server responds to the ping. Even when I try: ping 10.15I get a comparable result. Also, telnet to the partial addresses works as expected. However, SSH fails. Why do packets sent to a partial address arrive at the correct server?

Answer 1

That is an allowed form according to the inet_aton(3) function docs:

DESCRIPTION
       inet_aton() converts the Internet host address cp from  the  IPv4  num‐
       bers-and-dots  notation  into  binary  form (in network byte order) and
       stores it in the structure that inp  points  to.   inet_aton()  returns
       nonzero  if the address is valid, zero if not.  The address supplied in
       cp can have one of the following forms:

       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 net‐
                 work 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  rearrange‐
                 ment.

E.g.

$ perl -MSocket=inet_aton,inet_ntoa -E 'say inet_ntoa(inet_aton("10.0.15"))'
10.0.0.15
$ perl -MSocket=inet_aton,inet_ntoa -E 'say inet_ntoa(inet_aton("10.15"))'
10.0.0.15
$ 

However these days it would likely be better to instead use the getaddrinfo or inet_ntop calls for IPv6 support. The "Class B" stuff became legacy back in 1994 or so now that we have CIDR and /24...

Hey you can also give it a big old integer (but please don't)

$ perl -MSocket=inet_aton,inet_ntoa -E 'say inet_ntoa(inet_aton("2130706433"))'
127.0.0.1
$ getent hosts 2130706433
127.0.0.1       2130706433
$ ssh 2130706433
The authenticity of host '2130706433 (127.0.0.1)' can't be established.
...

(This may not be portable to other unix; in particular OpenBSD cannot resolve 2130706433...)