string_is_valid_hostname() returns true for IPv4 addresses
If it is meant to return true for all IP addresses, it should return true for IPv6 as well. If it's meant to return true just for hostnames, it should reject IPv4 addresses.
a valid host name can never have the dotted-decimal form #.#.#.#, since at least the highest-level component label will be alphabetic
Then we should check every use of this function, to make sure it is being used the right way.