AppendixD. Definitions of terms
TwoNTP servers communicating with each other use port 123 and the UDP protocol.
OpenWindows
OpenWindows is a window system that is used by several work stations. A similar window system is the X
WindowSystem, which Open Windows is based on. The X Window System and Open Windowsuse ports
6000and upward for traffic to the work stations. It is a good idea to block ports 6000-6010 for incoming traffic
froman unsecure outside network.
Packet
Whensomething 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.
Thereceiving computer has to reassemble the pieces.
Ping
Pingis used to examine whether a computer works and is accessible over a network. Ping sends ICMP traffic
tothe computer in question, and the target computer replies with a reply ICMP packet if it is running and
reachablefrom the network.
Youcan 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.
Theclient computer sends a type 8 ICMP packet, echo-request, to find out whether the target computer is
workingand 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
Whentwo computers are connected, ports are used. A client machine that wants access to a certain service on
aserver connects to the standard port for that particular service on the server. The programs on the client
machinereceive an available port over 1023. For example, if a user on the computer Tekla wants to run a
Telnetsession to the computer Winona, the user’s Telnet client program receivesan available port over 1023 to
connectto port 23 on Winona. If two server programs contact each other, one can act as a client program,
receivingan available port over 1023 on its local machine. However, many server programs havespecial
definitionsof how servers communicate with each other, where both servers user their standard port.
PPP
PPPstands for Point-to-Point Protocol. This is usually used to send IP packets over modem connections. See
alsoIP.
Protocols
Protocolsare sets of rules for how programs communicate with each other. For example, a web server can use
theprotocols HTTP and HTTPS.
Proxy
Proxiesare devices through which web pages, FTP files, and so on can be retrieved for a local network. This
canbe 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
pageinstead of fetching a new copy through the Internet.
Inyour web client, specify a computer and cache/proxy to be used to store this information.
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