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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