Administrator's Guide

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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