Esse é um formulário permitido de acordo com os documentos da função inet_aton(3) :
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.
Por exemplo,
$ 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
$
No entanto, atualmente, provavelmente seria melhor usar as chamadas getaddrinfo ou inet_ntop para suporte a IPv6. O material da "Classe B" tornou-se legado em 1994, aproximadamente, agora que temos o CIDR e /24 ...
Ei, você também pode dar um grande número inteiro antigo (mas, por favor, não)
$ 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.
...
(Isto pode não ser portável para outro unix; em particular o OpenBSD não pode resolver 2130706433 ...)