You'll want to think twice before disabling ports 23 and 515. Port 23 is associated with Telnet, so if you disable this port you won't be able to use Telnet any more. Port 80 is associated with HTTP (Web access) so if you disable that port, you won't be able to access any Web sites. 515 is the port associated with TCP-based printing services, so if you close that port, you won't be able to access the printer with TCP/IP.
Reader comments: From Jeremy Hines:
Just to let you know, port 9100 is used by some printing systems for TCP printing. Namely the AS/400 platform will talk to the JetDirect box on port 9100. Hope that helps to clear that up for you (I work in an AS/400 shop). From Diethard Ohrt:
I would like to add a comment to Ed Tittel's tip dealing with ports and HP JetDirect. This tip was about whether to shut down ports 23, 80, 515 and/or 9100. I want to add some information about port 9100:
In HP-UX, this is the default port for serial connections via hpnpf(1) (and maybe other utils).
hpnpf can be used to send files to a serial peripheral over TCP.
Port 9100 is also used for the serial port 1 (RS-232) on a X station (terminal); port 9101 is serial 2, 9102 is the parallel port.
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