Mac OS X 10.6: Adjusting advanced TCP/IP settings
You can use the advanced TCP/IP settings of Network preferences to adjust your TCP/IP settings for the network connection service you’re using, such as Ethernet, AirPort, or PPPoE. This may be necessary if your network connection requires you to set up an IPv6 address manually, or change your AirPort wireless TCP/IP settings from automatically receiving an IP address using DHCP to receiving it manually.
To adjust advanced TCP/IP settings:
Choose Apple menu > System Preferences, and then click Network.
Select the network connection service from the list, and then click Advanced.
Click TCP/IP, and then use the Configure IPv4 or Configure IPv6 pop-up menu to make the adjustments required by your ISP or network administrator.
If your service provider or network administrator requires it and you configure IPv4 or IPv6 using DHCP, enter the DHCP Client ID.