Arris SB8200
It has two network ports.
Thank you. The short answer is that only one should be used.
Long answer: Your ISP provides one IP address, so only one device can connect directly to the internet at a time—plugging in a second device (e.g., your computer and router simultaneously) will cause one to lose connectivity, as they compete for that single IP.
If you pay for two IP addresses (unlikely) then you could use both of the ethernet ports, each connecting out to the internet via a different public IP address.
Or, if you have an internet connection that provides 2 Gbps and a compatible network device you could use Link Aggregation to combine the two ports to access one public IP address at 2 Gbps. This is also very unlikely. A fast fiber connection to the internet like Verizon FiOS is typically 1 Gbps at best.
So, the Arris SB8200 has an extra ethernet port to accommodate arcane connection options that could occur in some comercial settings but for home use you would connect one and only one of the two Ethernet ports from the Arris to your router or gateway and switch (if using a more flexible setup from Ubiquiti).
I hope that helps.
If you have an idea of how many devices you will want to connect to the home network by Ethernet that could help define the best selection of networking equipment.
A long while ago I had an all-in-one home router box with the WiFi antennae in the same box as 6 ethernet ports. Everything works more reliably when connected via Ethernet vs. WiFi, so pretty quickly I exceeded the 6 port capacity of the router.
I then moved to a Ubiquiti Gateway, Switch, and WiFi access point. Getting a stand-alone switch let me get 20 ethernet ports and also move the WiFi antenna to a central location connected via an etehrnet cable to the switch. Moving the WiFi antenna got it away from the stereo equipment and provided better coverage. With a switch you can also have more than one WiFi access points to provide better coverage across a larger house.
