HP VCX Software manual 353

Appendix D. Definitions of terms

Open Windows

Open Windows is a window system that is used by several work stations. A similar window system is the X Window System, which Open Windows is based on. The X Window System and Open Windows use ports 6000 and upward for traffic to the work stations. It is a good idea to block ports 6000-6010 for incoming traffic from an unse- cure outside network.

Packet

When something is sent over a computer network, for example, a file or an email, it is divided up into sections. These sections are called packets. They make up a sort of jigsaw puzzle, each piece sent individually. The receiving computer has to reassemble the pieces.

Ping

Ping is used to examine whether a computer works and is accessible over a network. Ping sends ICMP traffic to the computer in question, and the target computer replies with a reply ICMP packet if it is running and reachable from the network.

You can also ping a whole network, and thereby use ping to examine which computers exist on a certain network. Therefore it is not advisable to allow ping into an internal network.

The client computer sends a type 8 ICMP packet, echo-request, to find out whether the target computer is working and accessible. The target computer ("server" in the picture below) replies with a type 0 ICMP packet, echo-reply, to tell it is working and accessible over the network.

Ports

When two computers use UDP or TCP to connect, ports are used. A client machine that wants access to a certain service on a server connects to the standard port for that particular service on the server. The programs on the client machine receive an available port over 1023. For example, if a user on the computer Tekla wants to run a Telnet session to the computer Winona, the user’s Telnet client program receives an available port over 1023 to connect to port 23 on Winona. If two server programs contact each other, one can act as a client program, receiving an available port over 1023 on its local machine. However, many server programs have special definitions of how servers communicate with each other, where both servers user their standard port.

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