until recently, My house has been serviced with a single modem, hooked up directly to the computer My dad uses for his business. recently, the family has convinced him to hook up wifi capabilities, which were mostly successful - except! that which provides the wifi cannot be directly connected to his computer (fears someone will hack into the network, just to get into his computer). Our option is to hook a new modem up without unhooking the old one. as such, my question is this:

Can two modems be hooked up to a single coaxial via splitter? Modem 1 provides landlines and Dad's PC, where Modem 2 will provide the Wifi for the rest of the house.

No, you can't do that. I mean, you could try, but ISPs generally configure the lines so that there can only be one customer per line.

Your father's fears are misguided: separating modems is not an effective way of fixing the "hacking" problem. What is this device that "hooks up wifi capabilities"? If it's a router, then he is already secured for the most part, since it includes a firewall. Just make sure to secure the router well (custom router configuration password, don't allow configuration from WAN, etc.) and make sure the PC is connected to the router, not the modem.

The no-no tendency that he's thinking of is perhaps connecting the computer directly to the modem. Depending on how sophisticated the modem is and its current configuration, the modem could either be using NAT already (such that every computer has a 192.168.x.x address), or be using bridged mode (such that every computer has an IP address assigned by the ISP).

If the modem is using bridged mode, then basically whatever device is connected directly to the modem is open to what is often referred to as "Internet noise" and this mode is intended for connecting a router to the modem.

If the modem is using NAT, then the modem is acting like a router, and all traffic is already being passed/filtered through the modem. Nowadays many modems can do Wi-Fi for you, so there's often no need to add an extra router to the network.

Lastly, your father needs to be sure he has a firewall enabled on his PC. Ask him to think, how exactly would a hacker "get in the computer"? Are remote desktop services disabled? Are file sharing services disabled? Is the firewall enabled? Are there any ports open? What kind of encryption is the router using? (If WEP, ask him to change it to WPA2 Personal/WPA2-PSK (AES), not WPA.)

If you can, can you explain the current configuration of the modem and router (default gateway and subnet mask for both)?

You assume my father can be persuaded; consider him a technophobe. If this doesnt work, then no, we dont get to change anything. Well, that stinks.

The current modem, given im not allowed to mess with it, i think it is running in NAT. Its a Cisco 3212, it cannot transmit wifi. The modem currently plugs directly to his PC.

No, im not going to ask him that, i'd much rather continue to be allowed to live under his roof a few more years.

edit:
Well, I could just get another new modem; what cox-compatible router provides phone landlines and at least 2 ethernet out ports?

I know my goal is to educate everyone (including your father) on these things, but sometimes it's necessary to stand back and let him shoot himself in the foot from his own stubbornness. Often times, the frustration that entails will result in him eventually turning to you for another solution.

For now, it's best not to intervene until he gives up and tries to consult you.

As oldmud0 said, "don't allow configuration from WAN".

Also, use an incredibly strong password for both the wifi and admin console; 12-16 random characters,numbers and punctuation marks. And a strong password would be pointless for wifi if it authorized/used WEP encryption, you must enforce WPA2.

WPA2 passwords can still be cracked, but it requires "brute-forcing" a captured handshake request (request to connect to the router which contains a hash of the password against a hash of the router's BSSID). Such attacks normally use massive textfile 'dictionaries' of common passwords (25GB is the largest i've seen), or rainbow tables (precomputed database of password list hashes, to cut down on computing time). Either way, a password that is truly random is probably your biggest defense against whoever your Dad thinks will hack you.

If you have comcast, make sure that the modem isn't broadcasting "xfinitywifi" as part of their "free public wifi service". I am not sure how it may or may not be segregated from the rest of your home network, but it might be worth looking into.

Also, depending on your wifi router's capabilities, you could also have a MAC address whiteless filter, which although not foolproof, thats just an extra layer of protection.