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<h2 class="first-section-title"><a name="255"></a><a name="ch08lev1sec1"></a>Key Terms and Concepts</h2><p class="first-para">An <b class="bold">IP address</b> identifies the location of a machine or device on a TCP/IP network. If your PC is on the Internet, you have an IP address. An example IP address is 192.168.1.101. Although it looks as if this address has four parts, it only has two: a network part and a host part. The network part identifies the network where the device is located, and the host part identifies the specific device within that network. In the example, 192.168.1 is the network part, and 101 is the host part. Most networks use the first three nodes of the IP address to identify the network. This is configurable, but that task is beyond the scope of this chapter. Most devices on a network are assigned IP addresses. Network communication between IP addresses is commonly called <b class="bold">traffic</b>.</p>
<p class="para">A <b class="bold">subnet mask</b> indicates which parts of the IP address identify the network and which parts identify the host. For instance, a common subnet mask is 255.255.255.0. This means that the first three nodes of an IP address indicates the network, the last node IP address indicates the host. As each node has 256 possible values (0 to 255), this example network can have 256 IP addresses in it.</p>
<p class="para">A <b class="bold">switch</b> or a <b class="bold">hub</b> is a device that allows multiple machines to participate on a network. There are technical differences between hubs and switches <a name="256"></a><a name="IDX-101"></a>that are beyond the scope of this chapter. A switch or hub can also be used to connect networks. For example, my PC is connected to a switch. That switch is connected to another switch that is connected to other machines in my house. Switches and hubs are devices that do not have assigned IP addresses.</p>
<p class="para">A <b class="bold">router</b> is a device that understands where to send traffic based on the network portion of the IP address. Routers are required for large networks. While hubs and switches allow you to create a network, routers connect entire networks. Some routers are programmable and can actually provide functionality similar to that of firewalls (defined later). Some operating systems are capable of making servers act as routers.</p>
<p class="para">A <b class="bold">gateway</b> is a router that provides users in a network access to the Internet. Gateways are typically provided by Internet services providers (ISPs).</p>
<p class="para">A <b class="bold">firewall</b> provides security for a network. The configurable rules of a firewall define what network traffic is allowed and block the rest. Firewalls keep Internet traffic away from servers that corporations use internally. Like many people, I have a firewall in front of my Internet connection to guard against security breaches. Here are a few examples of typical firewall rules:</p>
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<p class="first-para">Allow HTTP traffic from anyone to servers ren and stimpy.</p>
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<p class="first-para">Allow HTTPS traffic from anyone to servers ren and stimpy.</p>
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<p class="first-para">Allow FTP traffic from anyone to server homer.</p>
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<p class="para">A <b class="bold">load-balancing appliance</b> is used for Web sites with high volume. For Web sites that have high numbers of users, sometimes it's cheaper to buy several smaller servers than to buy one or two large ones. The load-balancing appliance distributes traffic over several identically configured Web servers.</p>
<p class="para">A <b class="bold">domain name service (DNS)</b> tracks labels for IP addresses. For example, it's easier to remember <a target="_top" class="url" href="http://www.javasoft.com/">http://www.javasoft.com/</a>, Sun's Java technology Web site, than to remember 192.18.97.39. A DNS tracks the fact that the Java technology Web site is at 192.18.97.39. A DNS also makes it easy for administrators to change the location of a Web site.</p>
<p class="para">A <b class="bold">demilitarized zone (DMZ)</b> is a network sandwiched between two firewalls. It's common to put a public site in a DMZ, with the Internet outside one firewall and a corporation's internal network outside the other. This decreases the probability of hackers getting into your internal corporate network because they would have to break through two firewalls to do so.</p>
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<p class="last-para">A <b class="bold">cluster</b> is a group of servers that service the same applications and are configured in such a way that they share one IP address. Clustering is a complex topic and can be defined at a hardware level, software level, or a combination of the two. Sometimes groups of servers like this are referred to as a <i class="emphasis">server farm</i>.</p>
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