Networking question (Comcast and DHCP)

[RipD]

Trying asking this on a networking discussion board and got no responses. Figured I'd try it here.

I have Comcast internet - no other viable choice where I live. I recently replaced their Netgear cable modem with a Motorola Surfboard (6121). Everything I'm reading tells me that the Surfboard should be in bridge mode rather than DHCP mode. This should happen automatically if the configuration is correct on the Comcast side. Comcast has no idea what I'm talking about and has a "your modem, your problem" attitude. Is there any way to get this into bridge mode? Any Comcast config that needs to change to make this happen?

Next issue is more logistical: I have a 3-story house. All my networking terminates in a top floor bedroom closet. It doesn't work well to have my wireless router there - I need it on the middle floor. So I'm kind of stuck running DHCP from Surfboard and putting my Asus router in AP mode, which is not ideal.

Any ideas? This can't be an uncommon problem, but I can't seem to find any solution to it.

Really hoping Google Fiber gets to my city next year (Google has said it's possible).

 

[Automata]

I would let the Surfboard router handle DHCP and run the wireless router in bridge mode (DHCP/NAT off). Is there a reason you don't want the Surfboard to handle DHCP?

This isn't an option that Comcast would change.

 

[RipD]

I would let the Surfboard router handle DHCP and run the wireless router in bridge mode (DHCP/NAT off). Is there a reason you don't want the Surfboard to handle DHCP?

This isn't an option that Comcast would change.

Yes. The reason is that in AP mode I lose a lot of the features of the Asus RT-N56U, like parental control. I want to use the features of the router, but can't do so. Another alternative is deal with double-NAT or perhaps disable NAT on the Asus. Not sure what the implications of that are.

Back to DHCP on the Surfboard, it's just wrong. It should not be in DHCP mode. The moto documentation says it's in DHCP mode when it can't get a connection to a provider. Something is clearly wrong here.

 

[Automata]

Double NAT will work fine unless you need to do port forwarding or need to access a system on the other part of the network. Port forwarding is do-able, but will be a pain since you need to do it on both routers.

Can you drop a cable down to the second story for the wireless router? If so, that might be your best option.

When the manual refers to "DHCP mode", what connection are they talking about? That could mean it listens for a DHCP response on the ISP line to get its address, or a DHCP server internally on your network. The latter does not make sense to disable for the home if it is the only router.

 

[RipD]

Double NAT will work fine unless you need to do port forwarding or need to access a system on the other part of the network. Port forwarding is do-able, but will be a pain since you need to do it on both routers.

So that's the big issue - I really want everything on one network. I have a lot of stuff that I'd prefer to run wired rather than wireless.

Can you drop a cable down to the second story for the wireless router? If so, that might be your best option.

Not easily. This was my first thought as well - just a dual network drop to one room on the middle floor and I'm good. No reasonable way to do that. What they heck are people thinking when the wire houses?

When the manual refers to "DHCP mode", what connection are they talking about? That could mean it listens for a DHCP response on the ISP line to get its address, or a DHCP server internally on your network. The latter does not make sense to disable for the home if it is the only router.

It's not mean to be a router. It only has DHCP so you can at least get an IP address if your cable is out. When properly connected to cable the DHCP is supposed to be disabled automatically.

It's perplexing.

 

[Automata]

That sounds like a really dumb router if true. If you want to keep everything behind one router, you will need to do exactly that. Since the Surfboard doesn't handle router duties, something else will. I'm not sure what to suggest.

 

[mimart7]

The wireless modems like the Surfboard, are all wireless gateways, so they cannot handle any of the other typical router duties.

 

[RipD]

The wireless modems like the Surfboard, are all wireless gateways, so they cannot handle any of the other typical router duties.

Not sure I'm following. The surfboard is just a cable modem. It doesn't have any wireless capability.

Let me try this again: I have a Moto Surfboard cable modem. I have an Asus RT-N56U wireless router. I have the Asus on the middle level of the house. All the network and coax cabling run the to top level of the house. I want to do two things:

1. Get my cable modem into bridge mode.
2. Find a way to get everything on one network. (see potential solution below.)

Possible solution: If I can get the Surfboard to do what it's supposed to, I think there's a pretty obvious solution. Run the coax for the cable into the room where I want the router so I have the modem and router next to each other. Then use the cat5 in that room to run the network back into the distribution box and then have a non-wireless hub there to distribute networking to the rest of the house.

The only small issue with this is that the room where I want the wireless router has coax on the other side of the room. (Really, what the heck are the guys who install this stuff thinking? This house is only 6 years old.) Other than an ugly coax run across the room, this should work.

How does everybody else do this? Am I missing something obvious?

 

[mimart7]

My mistake, I thought I remember the 6121 being wireless, I used to work for a cable provider, and part of my job was provisioning modems.

 

[Silver_Pharaoh]

Let the surfboard handle ALL DHCP requests. It will give out internal private IP address to your computers, there is absolutely no reason to disable DHCP period.

Now, the surfboard, how do you connect it to the ASUS? With hard wire or wifi?
If wifi, see if the ASUS can act in client bridge mode, it'll forward all DHCP to the surfboard and allow clients to wire into the ASUS for internet. (Maybe wifi if it has advanced features)

If it's wired, then it should be a simple matter of plugging the LAN cable from the surfboard into the internet (WAN) port on the ASUS no?



In my house I have my network like this:

Gateway modem --Wired--> Netgear Wifi router --Client Bridge mode--> DD-WRT Dlink router --Wired--> My computer.
Works fine, and both routers still have the parental controls and such. The netgear is simply off the shelf, just had to configure the SSID really, so it can do the parental control stuff, and the Dlink running DD-WRT can administer parental controls too. Doesn't make sense that the ASUS loses those capabilities when you put it into bridge mode.

 

[RipD]

Let the surfboard handle ALL DHCP requests. It will give out internal private IP address to your computers, there is absolutely no reason to disable DHCP period.

Now, the surfboard, how do you connect it to the ASUS? With hard wire or wifi?

Guess I'm not being clear. The Surfboard is a cable modem only. It's not a router or an AP. It has one input (cable) and one output (RJ45). It can only connected to anything wired.

If wifi, see if the ASUS can act in client bridge mode, it'll forward all DHCP to the surfboard and allow clients to wire into the ASUS for internet. (Maybe wifi if it has advanced features)

Yes. As I mentioned above I can put it in bridge mode (AP mode on the router) but then I lose most of the management features. [/QUOTE]

If it's wired, then it should be a simple matter of plugging the LAN cable from the surfboard into the internet (WAN) port on the ASUS no?

Yes. That's exactly how I have it now. I want to use the Asus as the router, not just as an AP.

Doesn't make sense that the ASUS loses those capabilities when you put it into bridge mode.

Agree. But that's how it works.

So I suppose another option is to get another wireless router that has more capability in bridge mode.

 

[Insane Scyth]

Hmmm, this is quite the interested case. The Surfboard modem is not supposed to be behaving in this way when connected to the Asus router. I have had this same setup awhile ago, and the modem was successfully handing my router a public IP, not a private IP.

My only suggestion at this point is to hook them up, log into the router, and if there is a private IP try release/renewing the IP. If that fails, I would look at making sure the firmware is up to date on both the modem/router then maybe testing another router to see if it behaves the same way.

What is the IP the WAN interface on the router is getting by the way?

 

[kyfire]

Where are you reading that the modem should be in "Bridge mode"?
I have read the entire manual for your modem ( http://www.arrisi.com/modems/datasheet/SB6121/SB6121_Installation_Guide.pdf and nowhere in it does it state this.
I would suggest you un-bridge thec modem, I have the order version Moto SB5120 and it's connected... cable>modem>router. All DHCP etc is handled by the router. And nothing is "bridged"

 

[Insane Scyth]

kyfire, thanks for linking that manual. Unfortunately, this modem does not have the ability to make many configuration changes. By bridge mode, he means the modem should be passing the public IP off to the router, not assigning a private IP causing double nat issues. That is how the modem should function, but it is not.

RipD, on page 15, you see the configuration for the modem. It might be worth resetting all defaults then restarting the cable modem.

 

[RipD]

What is the IP the WAN interface on the router is getting by the way?

It's a Comcast IP showing up on my Asus. So the bottom line is that it's working. I'm seeing conflicting info on whether or not the Surfboard should in be a DHCP server, but for now it's irrelevant since it's only providing one IP, and that's to the Asus.

I moved the Surfboard to the middle level of the house next to the router and then run a Router connection back to the panel. Once my new switch gets here I'll use that to feed connections to the rest of the house.

Thanks for the help.