Automatically Configure TCP/IP
Fortunately, you hardly ever have to actually configure TCP/IP properties for yourself. Instead, most network implementations today use a technology called Dynamic Host Configuration Protocol (DHCP), which automatically assigns IP address configuration information, including the subnet mask and (usually) default gateway. The process is as follows: your computer boots up, and as a part of XP initialization, the system contacts a DCHP server and requests an IP address. It does so because the TCP/IP Properties are configured by default to "Obtain an IP address automatically," as shown in Figure 9-4.
Figure 9-4. Obtaining an IP address automatically.
When this option is selected, your system is a DHCP client. Where is the DHCP server on your network? Well, it's probably your Internet Service Provider. If you have a persistent Internet connection, such as one from a cable or DSL connection, the ISP is your DHCP server. In other words, your Internet Service Provider typically assigns your TCP/IP address, and it could be a different address every time you reboot.
But DHCP servers can exist on other devices as well. If you're sharing out that same broadband connection with a router, your XP computer becomes a DHCP client of the router, and the router in turn is a DHCP client of the ISP.