As kacir suggested, I have tried to check
http://127.0.0.1:8080/ and seen Calibre management page.
itimpi:
Calibre main program has an icon on top right corner named "Connect/Share". Clicking upon it shows the option to "Start content server". Is that not the "content server" you have mentioned?
If using USB is not possible, could you please explain how I connect my iPhone to Calibre running on my linux computer "over the network" in order to synchronize my e-book library?
I have managed to send e-books to iPhone using e-mail as a temporary solution. However my purpose is sharing my Calibre library instead of sending books to iPhone individually.