Conform to RFC 1123 [0], specifically to "2.1 Host Names and Numbers",
which allow starting host name with alphanumerical instead of alphabetical characters.
RFC 1123 updates RFC 952 [1], which is referenced in "man 5 hosts".
[0]: https://tools.ietf.org/html/rfc1123
[1]: https://tools.ietf.org/html/rfc952