Mac Mini not connecting to internet

So now I am more confused, you are not on your own with confusion, same here.

OK one last try now I have a better idea of your network.

Using 'Network Utility' try and ping your router from the mini. Attached screen shot on my computer, use 10.0.0.1 as the IP address to ping. I have attached one that works to my router and one that fails to a non exsistant address on my network to show the difference between pass and fail.

If that works try the Lookup tab and try a domain that you know works with the Air, note this is the domain name not the website, as in my attached example, also attached a pass and failure.

There are other tests we can try but see how those work.

Leave the TC in Bridge mode, the 'connect using DHCP' just means the TC gets an IP address from the router which is good.

OK that means that no DNS servers could be reached, that could be two things.

1. The Mini cannot get past the router to the outside world.

2. The DNS servers you are using in the network setup of the Mini are wrong.

It is a little more difficult to ping outside servers to test conectivity due to many servers rejecting the ping command or ISP's not allowing them on their network.

The easiest thing next to try is make the DNS servers on the Mini exactly like the Air, same numbers same order. If the DNS does not cure it I am thinking some set up in the router is stopping the Mini, lets not go there yet.

To test the outside connection you could try to connect to Apple by its IP address just put http://17.149.160.49/ into the address bar, it is rather slow but if that works it is more than likey DNS at fault.

Dont forget to reset Safari if that is the browser you are using, before you try it to make sure you have no false readings.

The ipv6 addresses will be different, sorry I know nothing about IPv6, it will be voyage of discovery for me when it starts to be used on my ISP's network.

If the setup will let you disable IPv6 that would be worth try, I have seen it cause problems with stopping the internet working, I wonder if the Seagate isolated the IPv6 from the router and allowed it to work. Very easy to check if you can disable it.

Otherwise:-

You could try these DNS servers but it is looking as though for some reason you cannot get past the router. Did you try the IP address in Safari, it may only open part of the page due to the page being made up of many IP addresses.If the OpenDNS does not solve it, you may need to check your router to see if it is blocking the Mini in some way, that may be a call to Comcast.

https://store.opendns.com/setup/operatingsystem/apple-osx-snow-leopard

It points to Ethernet but do the setup on the Mini Airport settings.

You could also remove the Airport setting in the system Preferences and re add it to start a fresh configuration for the Mini.

Another few options to think about.

Did you remove IPv6 and try that? Or is it not possible, on earlier OS X it was, later ones did not have that option?Disable it in system preferences/Network where you reset up the airport, hopefully it is in the drop down list. Configure IPv6 is set to automatic at the moment.

Sorry I could not get the right answer for you, if you are near an Apple store you could try the Genius bar, thats free for them to check it out, not sure how much time they can give you but it may save you the Comcast fee if Apple can sort it.

Hope you get it sorted somewhere down the line, so frustrating for you and it is probably something quite trivial.