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New Visitor
highrise99
Posts: 4
Registered: ‎10-30-2012

Motorola SB6121 & DHCP

I would like to get a better modem and also save the leasing fee. I am considering the Motorola SB6121 and this one seems to have good reviews. I have a specific question about DHCP. My existing Arris modem can provide only 1 IP, so I am forced to connect it to my wireless router and then distribute IP to other devices. But I would prefer connecting the motem to a switch and then draw parallel connection to multiple devices.

 

Is the SB6121 having an inbuilt DHCP server and capable of providing IP to different devices that get hooked up via the switch? I have looked at many different places and don't seem to find a concrete answer to this question. Any help is appreciated. TIA.

Regular Problem Solver
Murphy
Posts: 470
Registered: ‎10-08-2003

Re: Motorola SB6121 & DHCP

Comcast does not allow you to have more than one IP address on a residential account.

A modem followed by a router is the only way to have multiple IP addresses.

 

New Visitor
highrise99
Posts: 4
Registered: ‎10-30-2012

Re: Motorola SB6121 & DHCP

Hi Murphy, my question was not about external IP and I know that I could get only one WAN IP. I am more interested to know if the modem would distribute the 192.168.100.x IP to the devices that are connected. If you look at modems like SBG901, they do the same thing allowing multiple devices to connect, though there is only one WAN IP

Networking Expert
kevj
Posts: 4,813
Registered: ‎10-03-2003

Re: Motorola SB6121 & DHCP

A modem is basically a passthrough device. The WAN IP you have is assigned to the interface on your router, and the modems are configured to only allow one MAC address to connect on the residential side. The WAN IP is associated to that MAC address. As far as I know, any modem that is configured using a Comcast configuration file does not distribute IPs in the 192.168.100.x network to internal devices, and approvedmodems connected to the Comcast CMTS will have Comcast config files pushed to them. Unpproved modems will not be provisioned.

 

The other point is that the SBG901 is not just a modem. It is a Gateway device (Combo modem/wifi router) which is why it acts as a DHCP server for internal devices, but I don't think it assigns 192.168.100.x addresses to internal devices....

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New Visitor
highrise99
Posts: 4
Registered: ‎10-30-2012

Re: Motorola SB6121 & DHCP

Thanks kevj. I might have been wrong with the IP range used within SBG901, but it obviously is some range either pre-defined or user defined. Let me ask the question from my intital post once again, and keeping it specific to the modem feature:

 

Is the SB6121 having an inbuilt DHCP server and capable of providing IP to different devices that get hooked up via the switch? The IP I am referring to is the LAN IP, either in the 192.x.x.x or 10.x.x.x range.

Networking Expert
kevj
Posts: 4,813
Registered: ‎10-03-2003

Re: Motorola SB6121 & DHCP

No....I do not believe so. If I am 'reading between the lines' of your question, it sounds like you want to connect devices internally using a switch. Is that correct? If so, if you connect a switch (not a router) to the cable modem you will only get internet access on one device, the one whose MAC address is associated with your WAN IP by the CMTS. You should be able to connect between other devices internally that are connected to the switch if they are all in the same IP network segment, but I don't think the cable modem will give out DHCP addresses to them, so you would have to statically IP all your other devices.

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New Visitor
highrise99
Posts: 4
Registered: ‎10-30-2012

Re: Motorola SB6121 & DHCP

You got it ! Yes, I am trying to run multiple devices via a switch connected to the modem. So, the SB6121 is no better than the one that I am having (apart from the speed factor). What modem would you suggest that would meet my requirement?

Networking Expert
kevj
Posts: 4,813
Registered: ‎10-03-2003

Re: Motorola SB6121 & DHCP

What you need is a router, not a switch...The SB6121 is a decent modem for Comcast service, but to allow multiple devices to connect as a LAN, you really need to have a router connected to the modem, not a switch. Consumer grade routers include either a 4 or 8 port switch, so you can connect multiple devices to them using ethernet cables, and the vast majority of consumer routers sold these days also have WifI capabilities, so you can connnect without a cable.

 

The router will also have an 'Internet' or WAN port, which is where the network cable from the SB6121 would connect. If you buy a router and connect it to the modem, please make sure you powercycle the modem so it can pick up the new MAC address that the router will present to it.

 

The router will provide the ability for all of the devices you connect to it, either wired or wireless, to connect out to the internet also.

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Connection Expert
EG
Posts: 42,443
Registered: ‎12-24-2003

Re: Motorola SB6121 & DHCP


highrise99 wrote:

 

Is the SB6121 having an inbuilt DHCP server and capable of providing IP to different devices that get hooked up via the switch? The IP I am referring to is the LAN IP, either in the 192.x.x.x or 10.x.x.x range.


FWIW, the built in DHCP server in a straight modem such as the 6121 only becomes enabled when the modem is not in block sync mode (no internet connection for whatever the reason may be) with the CC system. A basically useless feature IMO..