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I'm using Internet Sharing to share an Thunderbolt Ethernet connection over Wifi. Is there any danger of IP address conflicts with the devices the on the parent LAN?
Can devices on the parent LAN ping or directly communicate with the shared devices via their IP addresses?
In general yes and no. With your example of Thunderbolt in and WiFi sharing it's more likely you are in the "no" camp.
The yes is you are bridging more than one device in on the network connection and this warning is provided by Apple in the help:
WARNING: If your Internet connection and your local network use the same port (Ethernet, for example), investigate possible side effects before you turn on Internet sharing. In some cases, sharing your Internet connection disrupts the network. If you use a cable modem, for example, you might unintentionally affect the network settings of other ISP customers, and your ISP might terminate your service.
The no is when you share from one adapter to another you are being more safe since the DHCP/DNS effects of the added load on the main network since you are operating a new subnet and operating like a router as opposed to switching / bridging in all the connections to the main network.
In practice, you will get error messages and the network will fail should you overlap network segments and have IP conflicts. This is more likely if you specify static IP and netmask as opposed to letting DHCP handle things.
You'll also want to be sure you're using Private Network ranges (also known as non-routable or RFC1918) on IPv4. If you can move to IPv6 you will have more freedom and less chance of overlap but that's a bit more involved than a simple post here.
