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Appendix D. Definitions of terms
Two NTP servers communicating with each other use port 123 and the UDP protocol.
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 unsecure outside network.
Packet
When something is sent over a computer network, for example, a file or an e-mail, 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 are connected, 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.
PPP
PPP stands for Point-to-Point Protocol. This is usually used to send IP packets over modem connections. See
also IP.
Protocols
Protocols are sets of rules for how programs communicate with each other. For example, a web server can use
the protocols HTTP and HTTPS.
Proxy
Proxies are devices through which web pages, FTP files, and so on can be retrieved for a local network. This
can be good to combine with a cache memory, which will store pages and files once fetched from the Internet
site. When another user wants to look at a page already in the cache, it acts as a web server, sending the cached
page instead of fetching a new copy through the Internet.
In your web client, specify a computer and cache/proxy to be used to store this information.
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