See packet.


Dynamic Host Configuration Protocol. A protocol used by ISPs and some routers to automatically assign IP addresses to client computers and devices.

DHCP is most commonly used for establishing remote connections to the Internet. When a dial-up user logs on or when a cable modem user boots up their computer, a request is made to their ISP’s DHCP server. This request basically says, "Hey, I want Internet access!" After authorizing the request (typically with a username and password), the DHCP server picks out an IP address and leases it to the user’s computer, thus providing Internet access.

The same thing works on a smaller scale for Internet sharing setups; the server computer or router automatically assigns IP addresses to other computers on the LAN.


De-Militarized Zone. A common option on cable/DSL routers that allows one computer on the LAN to be placed outside the protection of the firewall. It is most often used to enable online games and other special applications that don’t normally work behind a router.


Domain Name System (or Service). Translates domain names into IP addresses. Every time you access a website, you computer sends the website’s address (domain name) to a DNS server. The server looks up the domain name in its database, finds the matching IP address, and replies back to your computer. In turn, your computer uses that IP address to contact the right server and access the desired page.

Example: when you enter "", your computer contacts your ISP’s DNS server, which responds with the IP address Your computer then contacts and requests the CableSense homepage.

This process is necessary because computers don’t understand names. "", for example, says nothing about the server’s location or connections. Instead, computers must use numeric IPs when it comes to organizing servers and routing information.

Note: Understanding DNS servers can be useful when troubleshooting an Internet connection. For example, if a computer can access but not, translation is failing and the DNS settings are almost certainly misconfigured.


Data Over Cable Service Interface Specification. The ITU standard for cable modems and related equipment, approved in March of 1998. All cable modems manufactured today and all current cable systems adhere to some version of this standard. As a result, any DOCSIS cable modem can be used with any DOCSIS cable system in the world.


A computer or group of computers represented by a single IP address. For example, if you share your Internet connection using a router or sharing software, all of your computers are considered to be in the same domain because they are using the same IP address.

domain name

A name assigned to an IP address or group of IP addresses. For example, "" is the name assigned to the IP address Since computers only understand numbers, domain names must always be translated into IP addresses before they can be used. A service called DNS performs this translation.


Refers to the transmission of data from the Internet to your computer. It is usually seen as downstream speed---the download speed of your Internet connection.


A software program that controls a physical or virtual device, such as a network card.


Digital Subscriber Line. A high-speed Internet service that uses regular telephone lines. Although it directly competes with cable modem service, technical limitations restrict DSL availability to within 18,000 feet of the local phone company’s central office.

DSL comes in several different flavors, each with a different speed and price. ADSL is most popular in North America, while SDSL has taken root in Europe. Although the service uses regular telephone lines, it does not interfere with telephone conversations because it operates on a different frequency that is easily separated by inexpensive line filters.


Refers to the ability to transfer data in both directions. Half-duplex allows for data transfer in one direction at a time; full-duplex allows for data transfer in both directions at the same time. Thus, if you have a 100Base-T full-duplex NIC, your theoretical bandwidth is 200 Mbps (100 Mbps in each direction). This is made possible becuase Cat-5 cable has separate wires for sending and receiving.

dynamic IP

An IP address that is assigned automatically by a DHCP server and is subject to change.

Most ISPs, including cable modem and DSL providers, use DHCP to lease this type of IP address to their customers. Cable/DSL users often think they have a static IP just because their IP never changes. In reality, most cable/DSL users really have dynamic IPs.

The source of confusion lies in cost reduction. Since dial-up users are online for relatively short periods of time, most ISPs never see more than 30% of their users at once. Also, since IP addresses are expensive, ISPs typically minimize the number of IPs they register. Logically, then, most ISPs only register enough IPs for 30% of their users. Each time a user signs offline, their IP address goes back in to the “pool” to be picked up by the next user. IP addresses are constantly recycled, so users rarely get the same one.

Cable/DSL users, however, are online a majority of the time. As a result, cable/DSL ISPs are forced to register IP addresses for nearly 100% of their users. With such an abundance of IPs, cable/DSL users usually just keep the same one they've always had. It’s important to realize, however, that “usually” is not a guarantee.