Question Unusal Cable Modum Router configuration question.

TheCasualObserver

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Dec 22, 2014
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Hi,

I would like to confirm a suggestion I received from an alleged former network engineer.

Here is the background:
My ISP is comcast. I have supplied my own (i.e. purchased) cable modem from the get go. My current system has been in place for a good number of years (10+, perhaps).

The requirements of my household have advanced quite a bit since the infrastructure was installed. I have been slowly upgrading. During a major remodel about 5 years back, I ran Cat 6 from the utility closet (where the cable comes in) to any room we thought we would ever need a connection. For wireless, I have an access point at the other end of the house; between the router and the AP I have decent to great coverage anywhere in the home. The only downside being the multiple SSIDs and explaining it to company.

I upgraded it last year to an Arris 8200 cable modem. Shortly thereafter my router bricked and I ran out and picked up an old (ancient?) belkin N300 router at the local electronic surplus store just to keep the household running through the holidays. Now I am finally shopping for a router that can keep up with the modem and the supposed bandwidth that I am paying the ISP to deliver.

The Question (finally):
In the past, just following the setup instructions I have configured the hardware as Cable Modem connected to router, router to household network.

Apparently, according to the expert I just met, I can just as well configure the hardwar as: Cable Modem connected to a Switch; Switch connected to household network; two of the lines coming out of the switch goes to my router and access point.

My reaction has been: wow, can that even work?? I'd like it to, gives my a lot more flexibility about where to place the router (actually the antenna on the router); but lots and lots of follow up questions come to mind:

What identity (i.e. the mac address) that is being presented to comcast? And what is the IP address that is being handed out and which device does the IP address belong to. These all may be the same question, just being asked from different perspectives - I don't know.

Doesn't the traffic on the lan cables double? And in fact doesn't household intra-net trafic leak out to the ISP?
 
Sound like you already know the truth. You only get 1 IP address from your ISP and it must be assigned to the router. If you hook the modem to a switch a end device could get the IP address and then nothing else would work. To work the traffic must flow from the modem to the wan port of the router and then from the lan ports on the router to the end device. The end devices must connect to the lan ports on the router.

Now if you got really fancy and used a switch with vlans you could hook the wan and lan port of the router to the switch and rig it so the lan and wan were different vlans. Technically you would be on the same switch but logically you would have 2 different switches one between the modem and the wan port and a second between the lan port and the end user devices.

But the more short answer is it must go modem---router--switch--devices.
 
As already ^mentioned, the standard is, your clients go BEHIND the router, the NAT device. Nat device allows multiple clients to use (typically) one public IP per household/simple site.

If u have reasons to hook things up other than standard, u are on your own as far as what non-standard config ur trying to achieve.
 

USAFRet

Titan
Moderator
It must be:
ISP->modem->router->switch and other devices.

The router is most of the brain here, in that it creates and manages all the internal IP addresses. DHCP.

The "expert" you just met is completely wrong.
 

digitalgriffin

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Jan 29, 2008
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I do something similar. The router would still handle dhcp request. However your router does do elementey firewall stuff. If you connect from modem to switch you lose that filtering. Also you need to plug your switch into the router lan port and not wan port.

The mac the service provider sees is just the cable modems.

That said the setup suggested is far from ideal because you dont have a firewall between you and your network. I would keep it cable modem->router wan->router lan->switch. Do not layer it any deeper. It creates issues the more hops you create. If you need more ports buy a switch with more ports. Dont chain it.

I can do this because i have two firewall appliances which act as my dhcp and firewall. So my router just becomes a fancy AP.
 

Greywolfe74

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Apr 20, 2019
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It must be:
ISP->modem->router->switch and other devices.

The router is most of the brain here, in that it creates and manages all the internal IP addresses. DHCP.

The "expert" you just met is completely wrong.
I have all my devices all plugged into the same switch, cable modem, router, and everything else. While it does require use of vlans, or other appropriate techniques, does not make it wrong. It's networking, and there's 10 millions ways to accomplish everything. Just because it doesn't match your single narrow minded belief, does not make it wrong, nor are you right. Your's is just one method, common, because of it's simplicity, and lack of need to understand how any of it works, to still function. When you add the appropraite sticky notes on the ports, we usually call yours the grandma setup.
 

USAFRet

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I have all my devices all plugged into the same switch, cable modem, router, and everything else. While it does require use of vlans, or other appropriate techniques, does not make it wrong. It's networking, and there's 10 millions ways to accomplish everything. Just because it doesn't match your single narrow minded belief, does not make it wrong, nor are you right. Your's is just one method, common, because of it's simplicity, and lack of need to understand how any of it works, to still function. When you add the appropraite sticky notes on the ports, we usually call yours the grandma setup.
The 'modem', in my chain of devices, speaks to one and only one device. That may be a PC, it may be a router.
But only one.

If it is a router, then you can hand other devices off it. A couple of PC, and also a switch and a bunch of other devices.
The router (or some other device) does the internal DHCP duty.

But a typical "modem" speaks to one and only one downstream device.
If you try to hang multiple devices off a "modem"...Please describe how a consumer grade modem (not modem+router) will serve multiple devices.

We await your flowchart.
 

Greywolfe74

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Apr 20, 2019
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You're remark was that the modem could only be plugged into the router first, and that into a switch first was wrong. And that the expert who said otherwise must be wrong. Well, you're wrong. Cable modem can indeed be plugged into the switch first, and everything else, firewall, pc, router, etc plugged into the same switch. It does require a different config(perhaps beyond your abilities), but it and the "expert" you attempted to belittle are not wrong.
And for the record, DHCP is hardly such a major function as to be the "brains" of a network. I currently run over 100 systems in my home, and I do not run DHCP services anywhere.

But if you like pretty pictures, then follow close. Cable modem<->3750E<->Nexus 3064<->Cisco UCS Fabric<->Firewalls<->Switch<->Core Router<->multiple switches/firewalls/network segments. FYI, that's three switches between the cable modem and anything else. Add logical in, and make it 4 switches counting the vDS.

As you can see, there's no reason a cable modem can't be plugged into a switch first. I just wouldn't give it to my grandmother to use.
 
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USAFRet

Titan
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Can it be done? Yes. If you want to go down the rabbit hole of increasingly complex setups.
For a typical home setup, the "modem" speaks to one and only one device. A PC or router.

In your setup, what device is doing the DHCP duty?
 

Greywolfe74

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Apr 20, 2019
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So you admit now, that your comment that everyone else must be wrong, and only your method is right, was incorrect? And frankly, I don't find that setup difficult at all, but 4 switches was just to show you how wrong you really were. As I already mentioned, I don't use DHCP. Its an unnecessary and superfluous service. Why do you think you have to run DHCPd?
 

USAFRet

Titan
Moderator
You do not have to run DHCP. Each device can have a static IP address, no problem.
Some of mine are static, some served via DHCP.

Unless you've contracted with your ISP for a bunch of public addresses, there needs to be some device that will serve those devices, though.
Again, far beyond the typical home setup.

From the OP's text:
"Apparently, according to the expert I just met, I can just as well configure the hardwar as: Cable Modem connected to a Switch; Switch connected to household network; two of the lines coming out of the switch goes to my router and access point. "

The 'cable modem' in this setup will simply talk to the first device it sees. All the others are out of luck.
 

Greywolfe74

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Apr 20, 2019
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Well sure, you need at least one valid IP for your connection to the internet. Sure you need a device, it's called the cable modem, or dsl, etc. But, there doesn't need to be DHCPd running anywhere on the network, nor is the modem excluded from being plugged into the switch first. My cable modem has to go through the switch first, since there's no telling where the firewall may have floated to in the network. But that gets into SDN, SDDC, converged infrastructure, etc, which is outside the scope of you being wrong in the first place. Next time, please consider that there are many ways to do things, and your simple setup can be an example of one way, it should not be taken as gospel for all.

As for "The 'cable modem' in this setup will simply talk to the first device it sees. All the others are out of luck."
That depends on how the switch(es) and/or devices are configured. Don't assume, and certainly don't claim its wrong when you have no idea.
 
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USAFRet

Titan
Moderator
Rephrasing my original statement:

"With the hardware you have listed, your expert is wrong"
or
"In a typical home LAN setup, it needs to be ISP->modem->router->switch and other devices"

Sure there are other ways to do it. Well beyond the scope of what the OP was asking for.

Clarification from both ends, and we both win...:)
 

Greywolfe74

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Apr 20, 2019
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Wasn't beyond what the OP was asking. He wanted to know, if he could do cable modem to switch, and place his other devices where they are more convenient. Very easily, with a simple, unmanaged switch, attach all components to the single switch. Assign appropriate IPs, and the OP, gets what he wants, to place his devices wherever he likes without limitations of grandmas setup "ISP->modem->router->switch and other devices"

Just because you don't know how to do it, don't tell everyone else, they're wrong, especially when you didn't even bother asking the details.

Example for OP(one way of doing it):
All devices, including LAN and WAN port of "router" plugged into switch. Use DHCP client on the router WAN port. Assign a network/subnet, such as as 192.168.1.1 255.255.255.0 to the lan port. Turn off DHCP services on the router device. Assign clients with a static address of 192.168.1.2-254 255.255.255.0 with a gateway of 192.168.1.1. Don't forget DNS, usually either your "router", or a public DNS on the internet. 8.8.8.8/8.8.4.4 for example(google).

See USAFret, yet another example of one way to do it that does not match yours. it's very simple, helpful, and constructive, versus what you originally posted. And not even running DHCPd either.
 
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