When you enable ICS, this enables the NAT protocol which is how you get through. NAT performs a network address
translation on packet destined to the Internet. It swaps out the private IP address of the source and plugs in the public IP
address of the ICS host and sends the packet to it's destination. ICS keeps a table of all these sessions so that when packets
come inbound, it can then perform the exact opposite process, removing the public destination IP and plugging in the private IP
of the destination based on the mapping table.
As far as clients go, ICS enables DHCP on the ICS host. Clients behind the ICS host will get a DHCP address which includes a
default gateway and DNS entries. The default gateway is what clients use to route traffic destined to the Internet through ICS.
This is how clients "Know" about the ICS machine. Clients really don't know that its a ICS host, they just know they have a route
out through this box. This is a extremely simplistic view of how it works but gives you the jist of it.
Thank you,
Mike Johnston
Microsoft Network Support
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