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Figure 6-6: This connection uses a static IP address instead of a dynamic one.
The more normal situation (and the default setup) for Windows XP Professional is to have Windows go fetch an IP address from another computer on the network whose job is to provide such dynamic addresses on demand. Such a computer is called a DHCP server (see 'DHCP, WINS, and DNS' later in this section).
Instant Answer Windows XP adds yet a third option, which is to try to fetch an address from a DHCP server, but to use a specific 'alternate configuration' (with a static IP address or an automatic Windows-assigned address) if no such DHCP server is around. Use the Alternate Configuration tab (viewable by clicking the Advanced button of the Internet Protocol (TCP/IP) Properties sheet) to configure this option.
The subnet mask is another four-number number, but it has a very specific function. Wrapped up in the four octets of an IP address are actually two addresses: the network ID and the computer or 'host' ID (kind of like a street name and a house number in a physical mail address). However, the dividing line between where the network ID ends and where the host ID begins is a flexible one; that is, it can change from one network to the next.
In order to figure out what exact part of any given IP address is the network ID and what part is the host ID, it's necessary to look at a second number that defines where the dividing line is for this particular IP address. That second number is - you guessed it - the subnet mask. TCP/IP uses the subnet mask to figure out, for example, whether a particular IP address is on the local network or on a remote network.
An example of a subnet mask is 255.255.255.0.
Instant Answer The bare minimum configuration information that a Windows XP Pro PC must have to participate in a TCP/IP network are
* IP address
* Subnet mask
Instant Answer If a PC's IP address and subnet mask are correctly configured, then the PC should be able to connect to a server on the local network.
Time Shaver You probably won't have to know the details of different classes of IP addresses, or how to interpret a particular subnet mask, for the Windows XP Professional exam.
The default gateway
If you want to connect to other networks, then you must configure TCP/IP to look for a gateway - a computer that links your network with another one (such as the public Internet). In this context, a gateway is simply a router, if you're already familiar with that term.
You can specify a default gateway on the General tab of the TCP/IP protocol's properties sheet, and you can specify additional gateways by clicking that properties sheet's Advanced button and entering the gateway IP addresses on the IP Settings tab (see Figure 6-7). The default gateway is the one your computer will use to connect to other networks unless the default gateway isn't available, in which case Windows XP tries to use a gateway that appears on the supplemental list.
Figure 6-7: You can enter one or more extra gateway addresses here.
Instant Answer You're likely to see more than one exam question expecting you to know that on a routed network, the proper default gateway for a client workstation is the IP address of the router's network port nearest that workstation - that is, on the same subnet as the workstation.
If you have a DHCP server on your network and you choose the Obtain An IP Address Automatically radio button (refer to Figure 6-6), then the DHCP server assigns your Windows XP PC a default gateway upon logon. If you specify a static IP address for the default gateway, the static address for the gateway takes precedence over the address that the DHCP server would assign.
Windows XP Professional only uses one default gateway at a time, so if you're connected to your company LAN and you dial up an ISP via a modem, Windows switches from the default gateway you've defined for your LAN to the default gateway specified by the ISP. That means that as long as you're connected to the dial-up account, you won't be able to see computers on your company LAN that are on a different subnet.
DHCP, WINS, and DNS
Hold out your bowl because here comes the alphabet soup. The programs in this section all have one overarching goal: to make managing IP addresses more automatic.
DHCP (Dynamic Host Configuration Protocol) is a service that assigns, or 'leases,' a computer an IP address from a range of IP addresses when that computer logs on to the network. Actually, DHCP also typically assigns a subnet mask, a default gateway address, and a DNS server address. DHCP can run on a Windows 2002 and 2000 Server, but it can also run on an NT server, a NetWare 4.11 or 5.0+ server, or a UNIX computer. The DHCP service does not run on Windows XP Professional or Windows 2000 Professional, although those machines can be DHCP clients. (A special case is when you set up Windows XP to use Internet Connection Sharing, in which case it runs a very limited version of DHCP, but still not the full-blown and configurable DHCP service that the server products use.)