Administrator's Guide

Appendix D. Definitions of terms
server. You can also establish an area of files that are accessible to others. Anyone can
log in as user anonymous and enter his email address as a password. They can then
access all files in this area, but nothing else. A computer with an FTP server and a
freely available area is usually called an FTP site.
Gateway
Gateway is an old name for a Router.
Hacker
A person who is skilled and knowledgeable about computers and likes to examine the
details of a computer system and what can be done with it. A hacker is good at pro-
gramming and achieves good results. A hacker is not to be confused with a computer
criminal; see Cracker.
HTTPS
HTTPS is WWW traffic (HTTP traffic) over an encrypted connection. The encrypted
connection is established using the SSL protocol.
ICMP protocol
ICMP is used to forward information, primarily error messages. To see if a computer
is running, the ‘ping’ program sends an echo request, which is an ICMP packet. If a
problem occurs with a connection, a response is sent using ICMP that something is
not right (the computer is not responding, the network is down, etc). If there are two
possible paths for a connection, a router along the way may tell the computer to use the
other path. The router sends an ICMP redirect. ICMP uses the IP protocol to send data
over the network.
IP address
IP addresses are used to connect to computers, and are the Internet equivalent to tele-
phone numbers. An IP address is divided into four groups, each of which is a num-
ber from 0 to 255. The groups are separated by dots. An example of an IP address is
192.165.122.42. Several IP addresses are required to connect several computers in a
network; one for each computer.
IP addresses can be divided into public and private addresses. Public IP addresses
are unique throughout the Internet, and can be reached by all computers connected
to the Internet. Private IP addresses can be used on several local networks, but can’t be
reached from other networks. When a computer with a private IP address wants to con-
nect to a computer on the Internet, the traffic must be NATed (see also NAT). See also
appendix C, Lists of Reserved Ports, ICMP Types and Codes, and Internet Protocols,
for a list of private IP addresses.
IP
IP is short for Internet Protocol. This is a protocol that is used to send data between two
computers on the same or different networks. IP performs no security checks. It works
analogous to standard mail. Peter sends four postcards to Christy from the other side of
the world. Christy gets postcard two first, then postcard one and postcard four. Postcard
three disappears on the way. Peter and Christy know each other’s addresses, and the
post office knows how to read addresses and send postcards in the right direction. But
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