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Networks that use the TCP/IP protocol rely on IP addresses to route packets of data from point to point. On a TCP/IP network, every computer has a unique IP address for each protocol (that is, TCP/IPv4 and TCP/IPv6) and each network adapter. An IPv4 address— the type used on most networks for many years to come—consists of four 8-bit numbers (each one represented in decimal format by a number between 0 and 255) separated by periods. An IPv6 address consists of eight 16-bit numbers (each one represented in hexadecimal format) separated by colons. In addition to the IP address, each computer’s TCP/IP configuration has the following additional settings:

A subnet mask, which tells the network how to distinguish between IP addresses that are part of the same network and those that belong to other networks.

A default gateway, which is a computer that routes packets intended for addresses outside the local network.

One or more Domain Name System (DNS) servers, which are computers that translate domain names (such as www.microsoft.com) into IP addresses.

To determine your computer’s IP address and other IP settings, click Details.

Windows Vista provides several methods for assigning IP addresses to networked computers:

Dynamic Host Configuration Protocol (DHCP) This is the default configuration for Windows Vista. Most internet service providers (ISPs) start with a pool of IP addresses that are available for use by their customers. ISPs use DHCP servers to assign IP addresses from this pool and to set subnet masks and other configuration details as each customer makes a new connection. When the customer disconnects, the address is held for a period of time and eventually released back to the pool so that it can be reused. Many corporate networks use DHCP as well to avoid the hassle of managing fixed addresses for constantly changing resources; all versions of Windows Server include this capability. The Internet Connection Sharing feature in Windows Vista includes a full-fl edged DHCP server that automatically configures all TCP/IP settings for other computers on the network. Most routers and residential gateways also incorporate DHCP servers that automatically configure computers connected to those devices.

Automatic Private IP Addressing (APIPA) When no DHCP server is available, Windows automatically assigns an IP address in a specific private IP range. (For an explanation of how private IP addresses work, see the sidebar “Public and Private IP Addresses.”) If all computers on a subnet are using APIPA addresses, they can communicate with one another without requiring any additional configuration. APIPA was first introduced with Windows 98 and works the same in all versions of Windows released since that time.

Static IP Addressing By entering an IP address, subnet mask, and other TCP/IP details in a dialog box, you can manually configure a Windows workstation so that its address is always the same. This method takes more time and can cause some configuration headaches, but it allows a high degree of control over network addresses.

Static IP addresses are useful if you plan to set up a web server, a mail server, a virtual private network (VPN) gateway, or any other computer that needs to be accessible from across the internet. (New features in Windows Vista such as Teredo and Windows Internet Computer Name make it possible to access a computer over the internet even without a static IP address, however.) Even inside a local network, behind a router or firewall, static IP addresses can be useful. For instance, you might want to configure the router so that packets entering your network on a specific port get forwarded to a specific computer. If you use DHCP to assign addresses within the local network, you can’t predict what the address of that computer will be on any given day. But by assigning that computer a static IP address that is within the range of addresses assigned by the DHCP server, you can ensure that the computer always has the same address and is thus always reachable.

Alternate IP Configuration This feature allows you to specify multiple IPv4 addresses for a single network connection (although only one address can be used at a time). This feature is most useful with portable computers that regularly connect to different networks. You can configure the connection to automatically acquire an IP address from an available DHCP server, and then assign a static backup address for use if the first configuration isn’t successful.

To set a static IP address, follow these steps:

1. In the Network Connections folder, select the connection whose settings you want to change.

2. Use any of the following techniques to open the properties dialog box for the selected connection:

> Select the connection and click Change Settings Of This Connection.
> Right-click the connection icon and choose Properties from the shortcut menu.
> Double-click the connection icon to open the Status dialog box and then click the Properties button on the General tab.

3. In the list of installed network components, select Internet Protocol Version

4 (TCP/IPv4) or Internet Protocol Version 6 (TCP/IPv6) and then click the Properties button.

4. In the Internet Protocol (TCP/IP) Properties dialog box, select Use The Following IP Address and fill in the blanks. You must supply an IP address, a subnet mask (for IPv6, the length of the subnet prefix, which is usually 64 bits), and a default gateway.

5. Select Use The Following DNS Server Addresses and fill in the numeric IP addresses for one or more DNS servers as well.

6. Click OK to save your changes. You do not need to reboot after changing your IP configuration.

To set up an alternate IP configuration, follow these steps:

1. From the Network Connections folder, open the properties dialog box for the connection you want to configure.

2. In the list of installed network components, select Internet Protocol Version 4 (TCP/IPv4) and then click the Properties button.

3. On the General tab of the Internet Protocol (TCP/IP) Properties dialog box, select Obtain An IP Address Automatically.

4. Click the Alternate Configuration tab and then select User Configured.

5. Enter the IP address, subnet mask, default gateway, and DNS servers for the alternate connection, as shown below. (You can safely ignore the fields that ask you to enter a preferred and alternate WINS server. WINS stands for Windows Internet Name Service, a name resolution system that maps a computer’s NetBIOS name to an IP address. WINS servers are used on large corporate networks to allow domain servers to communicate with computers running older Microsoft operating systems, including Windows NT, Windows 95, Windows 98, and Windows Me. For virtually all home and small business networks, the WINS server details are unnecessary and irrelevant.)

6. Click OK to save your changes. You do not need to restart after setting up an alternate configuration. When you’ve configured an alternate IP configuration for a network connection, Windows looks first for a DHCP server to assign an IP address automatically. If no DHCP server is available, the system falls back to the static IP address defined on the AlternateIP Configuration tab.