As I discuss in The DNS Server Settings On Your Computer, your ability to resolve server names into addresses is almost as important as the ability to contact the servers in the first place. The DNS client structure, which provides name resolution on your computer, allows for several possibilities.
If your Internet service goes thru a NAT router, you may be using the router as a DNS relay.
DNS Servers . . . . . . . . . . . : 192.168.0.1
Normally, as I discuss in the other article, you would not want a single DNS server. But if you have Internet service thru a single failure point (the NAT router), you might as well get DNS there too. If the NAT router goes out, you won't need DNS. Simplifying your setup makes sense here.
If you have a collection of computers, you can configure all of them to use the NAT router as an intermediary DNS server. The router maintains the actual DNS server relationships with its upstream feeds, checking the primary, secondary, even tertiary servers, as necessary. Each client has to worry about one relationship - the router.
But this can be a problem in one case. If your NAT router can be overloaded, it's possible that DNS relay functions may fail, while simple routing continues. The DNS relay function in ICS, if your Internet service depends upon an ICS server, appears to be subject to interruption when CPU load on the system is high.
This may be yet another reason why ICS is not a good solution for sharing Internet service.