Academic question about IP address assignment

Swampledge

Swampledge

Getting comfortable
Apr 9, 2021
218
474
Connecticut
My home network currently consists of an ISP-provided cable modem configured as a bridge only to my Netgear R7220 router/AP. connected to one of the LAN ports is an unmanaged switch which feeds two CAT6 cables, each serving a different outbuilding. One of those cables is connected to a LAN port of a Linksys E2500 router configured to serve as a access point only. The other cable is connected to a desktop computer. Pretty simple, right?

I set up the E2500 by connecting it via Ethernet cable directly to the computer, with the computer disconnected from my network, so I could log in to the config pages via 192.168.1.1. Once set up, I installed it in the other building, reconnected the desktop, and all was good. I searched for the E2500‘s IP address on my Netgear router’s device list, and couldn’t find it. No joy. A bit of research showed me that unmanaged switches don’t have/need IP addresses, so I concluded that the E2500 was seen by my router as an unmanaged switch and so didn’t assign it an IP address. (Maybe there just wasn’t a request from the E2500?).

Later, I rebooted the Netgear router as part of troubleshooting another issue. After doing so, my desktop had no Internet connectivity, because apparently, during the Netgear router’s brief ”absence,” the E2500 asserted it’s still assigned-but-unused 192.168.1.1 address. I fixed the issue by powering down the E2500.

Am I correct in concluding that, prior to rebooting the Netgear router, all communication between my network devices was via MAC addresses, so the IP address conflict between the Netgear and E2500 devices was irrelevant? If so, how did the desktop conclude that the 192.168.1.1 address correspond to the E2500‘s MAC address rather than the Netgear’s? Also, am I correct in thinking I can prevent future issues like this simply by assigning an IP address to the E2500?

TIA,
Jerry
 
The Linksys E2500 is not an unmanaged switch, it is a router. You may be using it as only an AP, however it is still a router, and on the LAN side there are a variety of manageable options that you may, or may not use.

You described: ISP-provided cable modem ---> Netgear R7220 router/AP ---> unmanaged switch ---> Linksys E2500.

Most all residential class routers default to an IP of 192.168.1.1 as the device address. Your two routers most likely have duplicate addresses.

Have you modified either the R7220 OR the E2500 to a different IP address?

Did you disable DHCP server on the E2500?

Which port on the E2500 did you connect the ethernet cable to?
 
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Yes, the DHCP server has been disabled on the E2500. As noted in the second sentence of my post, connection to the E2500 is via a LAN port, not the WAN port.

When I initially configured the E2500, after uploading newest firmware and a factory reset, I changed it’s IP address to one I knew wasn’t in use on my network. At that time, I connected it to the network and tried to access it via that address. I was unsuccessful, So I removed it and the PC from the network and tried again. Still no joy. So I did a 2nd reset and left out the step of assigning IP address. It’s possible that I forgot to hit save after the reassignment, or misremembered the address I had assigned, etc.
 
Please provide the LAN network config info (ip, subnet, gateway) for both routers

When trying to access the E2500, was it via wifi, or an ethernet cable. If it was via Ethernet, which router/port was it connected to
 
Swampledge said:
Thanks for the assist, but the reference you cited disagrees:
Click to expand...
That's exactly what I said to do. Plug the cable from the switch into the WAN port of the Linksys to use it as an access point. I have two routers I use as Access points in a mesh network and both have the cables from the switch plugged into the WAN ports.
 
All typical settings for a home network. Router serving as router, DHCP and DNS server, all set to 192.168.1.1. DHCP disabled on E2500. Only network connections via Ethernet cable between PC NIC and LAN ports of the 3 devices. I don’t understand what info you need that I’m not providing. Please keep in mind that I don’t need help solving a problem. I have the network working to my satisfaction. I am simply trying to understand better why my PC started to recognize the access point’s IP address instead of the router’s IP address during the brief period the router was restarting, particularly because there was no activity at the PC console. Maybe I’ll just attribute it to W10Pro phoning home nefariously.
 
brobin said:
That's exactly what I said to do. Plug the cable from the switch into the WAN port of the Linksys to use it as an access point. I have two routers I use as Access points in a mesh network and both have the cables from the switch plugged into the WAN ports.
Click to expand...
I believe when you do that you have also configured the WAN port to act as a LAN port.
 
No. When you plug the AP router's (Linksys) WAN port into the Main router's (Netgear's) LAN port the the main router assigns the AP router an IP address and anything connected to AP's LAN ports or WiFi gets an IP address from the Main Router. In the Linksys, DHCP should be disabled and the SSID set the same as the Netgear's. The instructions I referenced refer to "old" and "new" router. "New" refers to the Netgear and "old" refers to the Linksys being used as an AP. Hope this clears things up.
 
Thank you, brobin. That makes sense. If I understand you correctly, If I had connected the WAN port of the router being used as an AP, it would have been assigned an IP address by the DHCP function of the gateway router. Since I didn’t, that assignment never occurred, and all communication between the devices connected to the AP and the rest of the network was via MAC addresses. If I’ve got that right, I learned something.

I still wonder what caused the PC to update its MAC address table (if that’s what happened) when the PC shouldn’t have been looking for any device.
 
Now you've got it! The PC will look to whatever it's plugged into for an IP address. If the PC it was plugged into (or connected via WiFi) to the Linksys AP which had DHCP turned off, and the AP was not connected to the Netgear gateway router, then the PC wouldn't get an IP address. The Linksys was not connected to the gateway because the cable was plugged into a Linksys LAN port rather than the WAN port.
BTW, you can save yourself about $15/mo by replacing the ISP's rented modem with one of your own. It'll pay for itself in 3 months and save you $180/year!

Amazon.com: NETGEAR Cable Modem CM500 - Compatible with All Cable Providers Including Xfinity by Comcast, Spectrum, Cox | for Cable Plans Up to 400Mbps | DOCSIS 3.0: Computers & Accessories

Amazon.com: NETGEAR Cable Modem CM500 - Compatible with All Cable Providers Including Xfinity by Comcast, Spectrum, Cox | for Cable Plans Up to 400Mbps | DOCSIS 3.0: Computers & Accessories
smile.amazon.com
 
I would remove the e2500 and leave IP addressing to the first router. Connect the cat6 to another unmanaged low cost 4-8 port switch. the conflict will end. get a dedicated AP perhaps. things will be less muddy.
 
Once the reboot of the Netgear occurred, The computers IP address was no longer connected/valid/ or active, So the host computer was probably broadcasting a request on 192.168.1.255 for a DHCP server to give it an IP address. It was offered or negotiated a handshake with the Linksys and was given an IP address from the DHCP pool. because thats what routers do. If the computer had a static IP, it may have waited for the NetGear host to finish rebooting and then rejoined. The Linksys DHCP Pool might well include the PC's static IP address, and perhaps would not of been able to negotiate an agreement to provide a connection. Sort of like that.
 
Flintstone61 said:
Once the reboot of the Netgear occurred, The computers IP address was no longer connected/valid/ or active, So the host computer was probably broadcasting a request on 192.168.1.255 for a DHCP server to give it an IP address. It was offered or negotiated a handshake with the Linksys and was given an IP address from the DHCP pool. because thats what routers do. If the computer had a static IP, it may have waited for the NetGear host to finish rebooting and then rejoined. The Linksys DHCP Pool might well include the PC's static IP address, and perhaps would not of been able to negotiate an agreement to provide a connection. Sort of like that.
Click to expand...

Actually, that’s not how a typical dhcp client works. Just because the router rebooted doesn’t mean the dhcp lease is no good. Dhcp leases have a lifetime based on the server configuration for dhcp leases. Clients typically attempt to renew at the 50% point of the lease. The dhcp client will continue to attempt to renew (assuming the dhcp server is down) until the end of the lease at which time it will then release its IP and send a DHCP Discover packet

It’s at that point when the client technically has no IP address. Until that time, the client should still hold on to the IP for the rest of the duration of the lease.


Sent from my iPhone using Tapatalk
 
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Heres some basic how to info on using 2 routers. either Lan to Lan or Wan to Lan. It has some nice basic instructions.
www.wikihow.com

How to Connect Two Routers (with Pictures) - wikiHow

Plagued by poor Wi-Fi performance in certain parts of your home or office? Or just want to allow more devices to get online at once? If you have a second router that supports WDS, you can "bridge" it with your current router to expand your...
www.wikihow.com www.wikihow.com
 
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Lan to Wan. Lan is the output and Wan the input. Also just because it is in bridge mode does not mean that the webserver for config address on a unit is off. I have had several devices that were in bridge mode as far as traffic was concerned that still maintained a hard coded IP address so one could get to the config page without having to set it back to factory defaults. The thing that I have learned is to never use the default network range on the device except to set it up the first time. Keep a spread sheet or word document, something scribbled down with the changes you make so a year from now when you ask yourself how you configured it, you will know. Also if you add another LAN you will know what you have already used. Try to use address ranges that are uncommon too. It makes it easier to move between them or VPN. Do a google search on private IPV4 ranges if you don't know them by heart, And, as IPV6 takes over this will all change.
 
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