lighttpd: why using port >= 9000 does not work properly

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http://serverfault.com – I had a lighttpd server which works normally. I can access this website from outside(non-localhost) via http://vm.aaa.com:8080. Let's just assume that it's a simple static website, without php or mysql. Now I want to copy this website as a test one(using another port) in the same machine. And I do not want to use virtual host. So I just copy the whole files of original server, including lighttpd's bin/ conf/ htdocs/ lib/ and so on folders. And I made some required change, including changing lighttpd.conf. Now what I'm confused is, if change the port to a number that is less than 9000, it wor (HowTos)