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Contributor
Posts: 9
Registered: ‎12-24-2008
Accepted Solution

New Ultra customer with Motorola SB6120

I just converted from DSL to Comcast Ultra.  They installed a Motorola SB6120 cable modem.  I am only getting 10mb download through my Netgear Wireless Router.  The problem is a the router, because when I attach a PC directly to the modem, I get over 30 mb.  Does anyone know if the SB6120 cable modem can act as a Gateway and a DHCP Server?  I tried connecting the modem to my switch, but no one was getting DHCP addresses.  When I go to 192.168.100.1 under configuration I see this message.

 

DHCP Server Enabled
The SURFboard cable modem can be used as a gateway to the Internet by a maximum of 32 users on a Local Area Network (LAN). When the Cable Modem is disconnected from the Internet, users on the LAN can be dynamically assigned IP Addresses by the Cable Modem DHCP Server. These addresses are assigned from an address pool which begins with 192.168.100.11 and ends with 192.168.100.42. Statically assigned IP addresses for other devices on the LAN should be chosen from outside of this range

 

I can find no documentation on the SB6120 and no manual any where.  If this really can be used as a Gateway and a DHCP server, I can save some money.

 

- Mitch

Regular Problem Solver
cjortega
Posts: 953
Registered: ‎07-02-2003

Re: New Ultra customer with Motorola SB6120

[ Edited ]

mclaver wrote:

I just converted from DSL to Comcast Ultra.  They installed a Motorola SB6120 cable modem.  I am only getting 10mb download through my Netgear Wireless Router.  The problem is a the router, because when I attach a PC directly to the modem, I get over 30 mb.  Does anyone know if the SB6120 cable modem can act as a Gateway and a DHCP Server?  I tried connecting the modem to my switch, but no one was getting DHCP addresses.  When I go to 192.168.100.1 under configuration I see this message.

 

DHCP Server Enabled
The SURFboard cable modem can be used as a gateway to the Internet by a maximum of 32 users on a Local Area Network (LAN). When the Cable Modem is disconnected from the Internet, users on the LAN can be dynamically assigned IP Addresses by the Cable Modem DHCP Server. These addresses are assigned from an address pool which begins with 192.168.100.11 and ends with 192.168.100.42. Statically assigned IP addresses for other devices on the LAN should be chosen from outside of this range

 

I can find no documentation on the SB6120 and no manual any where.  If this really can be used as a Gateway and a DHCP server, I can save some money.

 

- Mitch 

 

"When the Cable Modem is disconnected from the Internet, users on the LAN can be dynamically assigned IP Addresses by the Cable Modem DHCP Server."

 

As the quoted line states, only when the cable goes down. This is so that if you do have multiple systems connected, they will be able to talk to each other.
 
The router might be connecting to the modem at 10 mb/s. Check the WAN port specs for the router, to see if it can do 100 mb/s.
 

 

Message Edited by cjortega on 12-25-2008 06:56 PM
Message Edited by cjortega on 12-25-2008 06:57 PM
Message Edited by cjortega on 12-25-2008 06:58 PM
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Claude
Contributor
Posts: 9
Registered: ‎12-24-2008

Re: New Ultra customer with Motorola SB6120

You are correct, I checked the SPECs of my wireless router.  Although the 4 switch ports are 10/100 the WAN port is only 10 Mb.

 

I still cannot figure out how to use the SB6120 as a Gateway.  I discounted the cable wire and did an IPCONFIG /RELEASE /RENEW and it looked like IP addresses were being assigned.  When I reconnected the cable no one had access to the internet.  This seems like a very poor process, that you need to discount the cable to asign DHCP addresses.  When I powered off/on the Cable Modem, all was lost.  I had to connect my old (slow) router.  Any other ideas on how to use the SB6120 as a Gateway.  Any recomendations on a good Router to get?

 

Thanks,

Mitch

 

Networking Expert
Baric
Posts: 24,238
Registered: ‎07-28-2003

Re: New Ultra customer with Motorola SB6120

[ Edited ]

You can't use the SB6120 as a gateway, it's only a cable modem.  It does have a DHCP server but that's only active if there is NO HSI connection.  In order to use a switch or hub directly connected to your SB6120, you need to pay Comcast for extra IP addresses.  By default, you get one and only on external IP address and your modem enforces that by the config file that Comcast downloades into it when it connects to Comcast.

 

The best thing to do here is simply replace the router, it's not up to the task due to its slow WAN port.  Get a model capable of 100Mbps on the WAN side, or better yet Gigabit ethernet.  I'd recommend a Linksys WRT610N.

Message Edited by Baric on 12-25-2008 10:17 PM
Contributor
Posts: 9
Registered: ‎12-24-2008

Re: New Ultra customer with Motorola SB6120

Thanks for your suggestion.  I went out and bought a new router.  Now most of my computers are getting over 40 Mb, but two only get 20 Mb.  All of my computers are connected to the same switch using Cat 5, and all show 100 Mb Ethernet and full duplex.  All of my computers have onboard NIC cards.  I wonder if the reason two of them are only getting 20 Mb has something to do with the onboard NIC card as I am not aware of any other differences, other than these are my two oldest motherboards.

 

- Mitch

Networking Expert
Baric
Posts: 24,238
Registered: ‎07-28-2003

Re: New Ultra customer with Motorola SB6120

It's hard to say, buy yeah, it's possible that the onboard NIC's are performing sub-par.  Lots of things effect performance, hardware, cabling, software like firewalls and virus scanners, etc.