AppendixD. Definitions of terms
Relay
Whenthe local network is connected to the Internet through a firewall, all types of services are usually
blocked.It is as if the network is not connected to the Internet. Relays can then be set up to allow certain
services,such as the WWW, to pass through under controlled circumstances. Think of it as a giant stone wall
witha gate and a specialized gate keeper. The gate keeper only lets certain visitors pass. To allow others to
passthrough, you set up another gate with another specialized gate keeper.
RFC
AnRFC (Request For Comments) is a document which standardizes some aspect of the Internet traffic. RFC:s
areavailable at http://www.rfc-editor.org/rfcsearch.html.
RIP
RIPis a protocol that manages dynamic routing. Dynamic routing means that the path for traffic can be
changed.RIP selects the path that goes through the least number of routers, but does not consider the
bandwidthor load on the network. RIP is only used in local networks. Fixed paths for traffic are called static
routing.
Router
Arouter is a machine that is used to connect several smaller and larger networks. Often, a router is used to
connecta local network to the Internet. This router only lets traffic to the Internet out; all other traffic remains
onthe local network. A router can also be called a gateway.
Routing
Arouting is a path for the traffic between different computers.
Server
Aserver can be a program that performs a service on a network or a computer that runs one or more server
programs.One example is a computer that stores files centrally, which makes it a kind of server, usually called
afileserver . The program that manages trafficso that people from the outside can access an organization’s
webpages is a server program.
SIP
SIP,Session Initiation Protocol, is a protocol for creating, maintaining and terminating various media stream
sessionsover an IP network. SIP is used to negotiate which media streams the parts can send and receive, and
whichparts should be involved in the exchange. When this is established, the media streams are sent according
totheir own protocols (e.g. HTTP). A thorough description of SIP can be found in RFC 2543.
SLIP
SLIPstands for Serial Line IP. This is usually used to send IP packets over modem connections. See IP.
SLIRP
SLIRPis a program that sends IP packets over serial connections, such a modem connections. SLIRP is run as
auser program. SLIRP does not need its own IP address; it uses the server’s IP address. The program works
withboth SLIP and PPP clients. See IP.
SMTP
SimpleMail Transfer Protocol, a protocol for sending e-mail between e-mail servers. SMTP uses port 25.
SNMP
Aprotocol used for network monitoring. SNMP uses ports 161 and 162.
Sockets
Whentwo computers connect to each other, they use their IP addresses and port numbers. The combination of
anIP address and a port number is called a socket. See IP addresses and Ports.
SSH,Secure SHell
SSHis a system for secure, encrypted connections between two computers over a network. SSH uses one open
andone secret key. In contrast to Kerberos, SSH does not use a central server for security. SSH uses port 22.
SSL
SSLstands for Secure Sockets Layer. The SSL protocol handles establishing of encrypted computer
connections.Usually HTTP and WWW traffic is sent on SSL. HTTP on SSL is called HTTPS.
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