httpd startup on non-standard port fails with error: (13) Permission denied: make_sock: could not bind to address 0.0.0.0:81

Issue

On boot-up and/or attempting to start the httpd service on a non-standard port fails with the following error:

# service httpd start
httpd: Could not reliably determine the server's fully qualified domain name, using 127.0.0.1 for ServerName
(13) Permission denied: make_sock: could not bind to address [[::]:81
(13) Permission denied: make_sock: could not bind to address 0.0.0.0:81
no listening sockets available, shutting down
Unable to open logs.                                    [FAILED]