If you don't know what your doing and looking for the best modem to get, buy this one. It's the best Arris Surfboard modem out. Spend the extra money, it's worth it. This one also supports DOCSIS 3.1, so it's future ready.
Word to those who don't know. DO NOT use modem/router combos (even the Arris SB ones)! Buy a modem only, and a separate router! In today's world, everything Gigabit.
Modem: Arris Surfboard - SB8200
Router: Linksys WRT1900ACS v2 (flashed with DD-WRT)
Switch: Netgear 16 port Gigabit Switch - GS116NA
I bought this modem to replace my 4 year old Motorola/Arris SB modem (looks almost just like this one). I should have gotten a little more life out of it, but I am ok with the 4 when you factor possible power surges and stuff on my home end, and the rate of necessary technology upgrades. It was near time to upgrade anyways.
My speed tests (speedtest.net) max out my internet speeds. I didn't notice any speed difference, as my previous modem was also a surfboard. It was expected to be the same. Though, I know I am more future ready and my modem is capable of handling whatever the next speed upgrade is to my internet. You don't want to buy a new modem to only have to buy another one a year or two later because it can't handle your new package speed.
This modem can handle Gigabit internet packages!
Directly connect a computer via Ethernet to your modem. Do not go through your router yet. Straight modem to PC. Power cycle the modem, wait for it to fully connect (solid blue light 2nd from top). With Comcast, you will get redirected to their activation page. Enter your info, and once it's fully done and activated, you should be able to get on the internet.
Go to speedtest.net and run a speed test. Screenshot it for your records. If all is good, hook your modem up to your router. You will need to power cycle the modem again, as it's "locked" to the first device on the network. Once back up, connect to PC to your router with an ethernet cable. Do another speed test and take a screenshot. It is normal to see a slight decrease as your going through another device.
If you connect via wifi, you will have a huge decrease, which is why I recommended to use ethernet to the router for the speed test part. When your all done, connect to wifi and all that as normal :) 5ghz is faster than 2.4, remember that if you do a speedtest on wifi. Even 5ghz will be way slower than wired. So don't do a speed test on wifi and complain to your ISP that your not getting your full speed - your not going to!
PS: You should be using Cat-6, not Cat-5 ethernet cables.