do i have to buy the netgear ac1200 wi-fi usb and if so what does it do ?

First, a brief lesson on the Netgear AC1750 Wi-Fi docsis 3.0 - this item combines your Internet providers cable modem and your internal Wi-Fi router into 1 piece of equipment instead of having 2 sitting on your desktop; however, the more important feature of this item is that you won't need your cable providers cable modem (which I pay $7 per month to rent - there are some pros and cons to this because if it breaks you are on your own if you own the equipment, if you rent it for $7 per month the cable company will replace it for free). Now the other feature of the Netgear Ac1750 Wi-Fi docsis 3.0, which is now getting into more of the question you asked, is that it provides Wi-Fi (wireless network/internet access) to your house. In order to be able to use the Wi-Fi feature, you must have something on the PC/TV/Roku/Amazon Fire Stick/etc. that can talk back to the netgear ac1750, that something (sometimes called an adapter) is what you are asking about. So now let me explain something about Wi-Fi before I answer your question. Wi-Fi has different levels of speed/fastness/throughput they are categorized by letters b/g/n/ac. For simplicity sake, b and g are the slowest, n and ac are the fastest. This router provides 2 access points, the first one for b/g/n and the second one for n/ac. Most laptops/pc/tvs/etc can access the routers b/g/n access point because this technology has been around for a long time and is already built into those devices; however in order to take advantage of this routers fastest and latest and greatest technology (ac), you would have to have the ac technology built into your pc or use the Netgear ac1200 Wi-Fi usb (the item you are asking about). So to answer your question, you do not have to buy the netgear ac1200 to use the router (because you can still use the b/g/n or n/ac Wi-Fi access points for your older equipment). I just bought a new laptop and it had the ac technology already built into it. If you are connecting your pc/equipment hardwired to the netgear ac1750 Wi-Fi docsis 3.0, you won't need anything except the network cable to attach it (the Wi-Fi speed would not apply because it is hardwired, hardwired is always the fastest way to go).

does this work with time warner

Yes. Google "your provider recommended modems". We're on Comcast but may be moving and we have found the C7000 on the recommended lists for Comcast (the box has "Xfinity" printed on it, Time Warner and Cox so far. There may well be others we have not checked.

will ooma work with NETGEAR AC1750 Wi-Fi DOCSIS 3.0 Cable Modem Router (C6300)

Yes, we have ooma connected by Ethernet cable and it works fine, with Comcast as the internet service provider.

Will this replace Xfinity rental if I get voice service as well?

The Netgear C6300 only has 4 Ethernet ports for networking, and no ports for telephone, unless Xfinity has a separate box for telephone. Usually, Xfinity or other providers have a combo modem router with phone ports (EMTA modem routers or EMTA modems that connect to a network router). I worked for Cox Communications, and you must have a special EMTA modem or EMTA modem router for phone service from Cox, and that's the same for other phone and Internet providers.

Brand	NETGEAR
Product Dimensions	17.4 x 27.79 x 7.52 cm; 9.07 g
Item model number	C6300-100NAS
Manufacturer	Netgear
Series	C6300
Color	Black
Connectivity Type	Wi-Fi
Wireless Type	802.11a/b/g/n/ac
Number of Ethernet Ports	5
Voltage	12 Volts
Hardware Platform	PC
Operating System	Linux
Are Batteries Included	No
Item Weight	9.07 g

NETGEAR Cable Modem WiFi Router Combo C6300 | Compatible with Cable Providers Including Xfinity by Comcast, Spectrum, Cox for Cable Plans Up to 400Mbps | AC1750 WiFi Speed | DOCSIS 3.0

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