Depends on cable modem. If the modem is set to put the LAN ethernet connection back onto the cable, you need only one. You should be able to have the modem do that. I am using only one adapter now (ECB2200V) to put my A/V stuff on the network.
"Music in the Round"
Senior Contributing Editor, Stereophile
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The configuration I use does not require "pairs" of MoCA adapters within the context of a Verizon FiOS household.
For mine, I utilized the Netgear MoCA COAX-Ethernet adapters:
At the coaxial wall connection where you are wanting an Ethernet drop, I connect the input side of a bi-directional 2-way splitter, to the coax wall connector (5-1000MHz is the frequency you want).
Notes: do not use coaxial amplifiers anywhere upstream as they can block the necessary frequencies to support the TV signal, as well as the Ethernet signal.
If you wish to keep it "clean" at the wall, you could use one of these types of splitters instead:
From the 2-way coax splitter, one of the output feeds goes directly to the Verizon FiOS set top box. The other output feed goes directly to the MoCA adapter "Coax In". I do not use the "Coax Out" connector on the Netgear MoCA adapter.
On the Netgear MoCA adapter, there is an Ethernet port. You can either plug an IP device directly into this port (e.g. TV, blu-ray player, streaming device such as a Roku, game console, etc.), or you can plug in an Ethernet hub to provide more ports. If you use an Ethernet hub, there is typically a designated "Uplink" port, or one of the ports to make this connection (see manual).
I also use a MoCA POE (Point of Entry) filter on the main Verizon feed coming into the house (the coax coming in from the "street", that goes into the Input side of the main coaxial splitter in your house). You can find a POE here:
Mode links to help you here:
http://forums.verizon.com/t5/FiOS-TV-Technical-Assistance/Splitters/m-p/7106/highlight/true#M3644 (discussion on correct splitters)
Hope this helps!