This URI is simply an IPP standard, you will always need to specify /printers/printername to access a printer via IPP. Once you installed a printer you can watch the URLs in your web browser as you are working with the web interface of CUPS – you will see that you manage your printer by navigating to that exact same URL: localhost:631/printers/printername. The Internet Printing Protocol uses port tcp port 631, so make sure that this port is not blocked by an installed firewall – especially Redhat blocks most ports by default. See http://www.cups.org/ipp.html#3_1 for details.