User guide

Glossary
EN/LZT 108 6492 R1 93 (98)
April 2003
DMZ
A Demilitarized Zone is used by a company that want to host its own Internet services
without sacrificing unauthorized access to its private network. The DMZ sits between
the Internet and an internal network's line of defense, usually some combination of
firewalls and bastion hosts.
DNS
Short for Domain Name System (or Service), an Internet service that translates domain
names into IP addresses. Because domain names are alphabetic, they're easier to
remember. The Internet however, is really based on IP addresses. Every time you use
a domain name, therefore, a DNS service must translate the name into the
corresponding IP address. For example, the domain name www.example.com might
translate to 198.105.232.4.
The DNS system is, in fact, its own network. If one DNS server doesn't know how to
translate a particular domain name, it asks another one, and so on, until the correct IP
address is returned.
Domain name
A name that identifies one or more IP addresses. For example, the domain name
microsoft.com represents about a dozen IP addresses. Domain names are used in
URLs to identify particular Web pages. Because the Internet is based on IP addresses,
not domain names, every Web server requires a Domain Name System (DNS) server
to translate domain names into IP addresses.
Downstream
The direction of a downstream signal is from the ISP/service provider to the user's
computer (downloading).
DSL
Short for Digital Subscriber Line, which is a data communications technology that
transmits information over the existing copper telephone lines (POTS). DSL takes
existing voice cables that connect customer premises (CPE) to the phone company's
central office (CO) and turns them into a high-speed digital link. There are many
types of DSL and ADSL is one of them.
DSLAM
Short for Digital Subscriber Line Access Multiplexer, a mechanism at a phone
company's central location that links many customer DSL connections to a single
high-speed ATM line.
When the phone company receives a DSL signal, an ADSL modem with a POTS
splitter detects voice calls and data. Voice calls are sent to the PSTN, and data are sent
to the DSLAM, where it passes through the ATM to the Internet, then back through
the DSLAM and ADSL modem before returning to the customer's PC.
Ethernet
A local-area network (LAN) architecture that uses a bus topology and supports data
transfer rates of 10 Mbps. It is one of the most widely implemented LAN standards.
A newer version of Ethernet, called 100Base-T (or Fast Ethernet), supports data
transfer rates of 100 Mbps. And the newest version, Gigabit Ethernet supports data
rates of 1 gigabit (1,000 megabits) per second.
Firewall
A system designed to prevent unauthorized access to or from a private network.
Firewalls can be implemented in both hardware and software, or a combination of
both. Firewalls are frequently used to prevent unauthorized Internet users from