AppendixD. Definitions of terms
192.165.122.42.Several IP addresses are required to connect several computers in a network; one for each
computer.
IPaddresses were previously divided into A networks, B networks and C networks, but that terminology is
nowconsidered obsolete. An A network was one where the first group of numbers is predetermined and you
determinethe remaining groups yourself; for example 17.x.y.z . A B network was one where the two first
groupsare predetermined; for example 128.42.y.z. A C network was one where the first three groups are
predetermined;for example 192.168.12.z .
IP
IPstands for Internet Protocol. This is a protocol that is used to send data between two computers on the same
ordifferent networks. IP performs no security checks. It works analogous to standard mail. Peter sends four
postcardsto Christy from the other side of the world. Christy gets postcard two first, then postcard one and
postcardfour. Postcard three disappears on the way. Peter and Christy know each other’s addresses, and the
postoffice knows how to read addresses and send postcards in the right direction. But Peter and Christy cannot
knowif all of their postcards will arrive. And Christy doesn’t know what order the postcards were sent in.
Formore information about IP addresses, see IP address.
Kerberos
Kerberosis a system to secure connections between several computers over networks. The Kerberos system
usesa Kerberos server to manage security. Connections that go through Kerberos are often encrypted.
Masquerading
SeeNAT.
Nameserver, DNS
ADNS server is the Internet equivalent of dialing telephone information. If you know the name of a computer,
youcan access its IP address and vice versa. The server keeps track of names and IP addresses. Imagine that a
userwants to connect to the computer "Tekla" through a Telnet (terminal) connection. The Telnet program
asksthe name server about Tekla and receives Tekla’sIP address. If the name server does not know a name, it
asksthe nearest name server. See the figure.
Nameservers are usually named primary,secondary, or other .If you have several networks with several name
servers,they can communicate with each other. It is a good idea to make them secondary name servers to each
other.Secondary name servers work as extra name servers if the primary server is not working.
Asecondary name server updates its information from the primary name server at regular intervals. You can
specifyhow often. Only the manager of the name server can set it up as a secondary name server for someone
else.In the figure below, we have two local networks with separate name servers. If name server Amanda does
notwork, a machine in network 1 may ask the name server in network 2, Bertha, if this server is set up as
secondaryname server for Amanda. Other name servers outside network 1 and 2 belong to the other category.
Thename server responds to name queries on port 53. Both TCP and UDP is used for name queries.
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