JdeBPEverything that can accept human-readable IPv4 addresses in place of hostnames, and that does so by using the conventional C library inet_aton() or inet_pton() functions.
A single (32-bit) number has been an accepted human-readable form from the start, back in the 4.3BSD days. It's a thing that gets regularly rediscovered.
(Bernstein's networking tools use a ip4_scan() function that only supports strict 4-decimal-numbers form. But several of them special case and re-map the string "0" before calling this function. libowfat's scan_ip4() is similar to ip4_scan(). As is ip4_scan() in @ska's libstddjb.)
This and three other accepted formats are explained in all of the #FreeBSD, #OpenBSD, #NetBSD, and Linux GNU libc manual pages. They also explain why 0x00000000 and 0x7F000001 and 0177.1 will work as IPv4 addresses with such programs.