Question Static IP/Should DHCP be enabled on cable modem?

May 2, 2019
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I have a Spectrum Cable Modem setup with a Static IP Address so that I can RDC to my customers. I have Cable Modem going to switch, then that is plugged into eero WiFi router. I have two PCs plugged into switch via Ethernet. All WiFi devices are working. Suddenly my PCs stopped getting Internet. When I plugged directly into cable modem, I get Internet, but not my Static IP. Spectrum confirmed the modem is broadcasting correct Static IP. This worked fine for 18 months.

Why did the PCs stop connecting to Internet at the same time? Should the cable modem be DHCP enabled? My PCs are setup to automatically get IPs. Thanks!
 
You have to be very careful what you call devices. If the device you are calling a modem has DHCP then it is a router. A modem in almost all cases can not be hooked to a switch like you have done. The ISP generally only gives you a single IP address and it will be assigned to the first device you attach and no other device will get ip addressses.

Now in your case if you pay for something like a static ip maybe you also are allowed multiple ip addresses. Since this is no the common install I can not even guess what is going on.

So first you need to determine if the device is actually a modem or a modem/router.
 
May 2, 2019
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It is a modem router. I pay extra for a Static IP. Spectrum can’t tell me if DHCP should be enabled or disabled on their modem router.

Both PCs have shown the same outgoing Static IP for 18 months and then both stopped working. They gave me a new modem router today, but still PCs are showing wrong IP, not the Static one I pay for. Thanks for the help.
 

digitalgriffin

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Jan 29, 2008
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I have a Spectrum Cable Modem setup with a Static IP Address so that I can RDC to my customers. I have Cable Modem going to switch, then that is plugged into eero WiFi router. I have two PCs plugged into switch via Ethernet. All WiFi devices are working. Suddenly my PCs stopped getting Internet. When I plugged directly into cable modem, I get Internet, but not my Static IP. Spectrum confirmed the modem is broadcasting correct Static IP. This worked fine for 18 months.

Why did the PCs stop connecting to Internet at the same time? Should the cable modem be DHCP enabled? My PCs are setup to automatically get IPs. Thanks!
What the isp gave you is a static WAN IP address. However your LAN IP address is different. That number is assigned by the dhcp of whatever router you are using.

To connect remotely to your clients (remote desktop connection), you don't need a static IP. since you are connecting to them they are the ones that need a static ip.

The only reason you would need a static Wan IP is if they are connecting to you to access some service on your network. (VPN, FTP, Web Server, etc...). Even then with a dynamic IP on your cable modem, you can use services like OpenDNS to direct you to your router (provided your router supports such services)

WAN IP addresses are expensive and there's a limited number of them (4 byte limit) This is why they are often reshuffled around. A WAN IP is what the ISP assigns to your cable modem. So the IP assigned to you is WAN IP address to the outside world. Your internal network uses a LOCAL IP which you designate in the router setup.

DHCP means the router assigns a RANDOM UNUSED LOCAL IP address to your machines as they request them. So you might boot up your computer one day and it reads 192.168.1.128. Another day it may read 192.168.1.196. If you are doing port forwarding for a web service, this will not work. If you are using Port Forwarding to forward services from the outside world, you need to assign a static IP to the machine using the machine's MAC. This way your IP address doesn't change. All network hardware and servers on my local network, as well as printers use STATIC ASSIGNED LOCAL IPs. In fact all 40+ devices on my network have static LOCAL IPs. It makes it easier to detect intruders.

I also do not recommend your clients get unrestricted access to your local network (VPN, FTP) because there is potential for mischief. And Web servers (Webpages/WebAPI's) are controlled by experienced programmers which limit the clients ability to create mischief.



When traffic comes in via your cable modem, the incoming packets get reformed through the NAT (Network Address Translation) This is your local IP address. These are your private IPs like 192.168.x.1.

The first device on your network AFTER your cable modem should be the device which handles the NAT and the DHCP request. Not the switch. Everything that comes out of your cable modem is unfiltered WAN (internet) traffic. That means your devices will not be protected by basic NAT filtering and common firewall rules of the Router. (Even though their subnets are likely to be different)

If you need more ports than your router provides go:
cable modem->router WAN->router LAN->Switch->Device Clients

Put your higher traffic/higher priority devices directly attached to the router. For example, my Streaming server is connected directly to the router. This avoids excess "hops"
 
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