Windows ICS(Internet Connection Sharing) is actually a simple gateway and DHCP server. All operating systems that support TCP/IP and DHCP protocols can be a client of Windows ICS. For example, FreeBSD.

One way to configure a FreeBSD as an ICS client is to use the "sysinstall" command on root account. In the menu of sysinstall, select Configure->Networking->Interfaces->(your ethernet card). Then you will be asked two questions. One is about IPv6, and the other is about DHCP client. Please answer yes to the second question. Then you can get your local IP, usually in the form 192.168.0.x, from the ICS server. Save your configuration and exit sysinstall.

After using the sysinstall, you can find a few lines are added to the file /etc/rc.conf. So another way to set a FreeBSD ICS client is to manually add these lines to /etc/rc.conf.