Router & Modem WAN IP address questions

videobruce · Post by **videobruce** » Sun Dec 31, 2017 10:05 am

Trying to understand the events involved for a Router to get recognized with a WAN IP address thru a Modem. This is with a CATV ISP (Spectrum/TWC if that matters)

Flashing the firmware, or any activity that requires disconnection of the link between the Modem & the Router.
Modem & Router on line,
WAN cable disconnected from Modem to flash Router,
After flash, WAN cable reconnected, connectyion to Modem shows active via LED indicators on both devices,
PC can access Routers web interface ok, and clinest get populated ok also (they have static IP address set within the devices),
Router does not have a WAN address, any Gateway or any DNS address from the ISP yet.

The "DHCP Release & Renew" function in the Router isn't that suppose to solve this without rebooting the Modem?

Post by **Philip** » Sun Dec 31, 2017 3:27 pm

I am not sure I understand your question.

Many modems also act as routers, so they do have a built-in DHCP server and give out IP addresses in the private ranges.
If you connect a router to a modem without using the WAN port on the router, you are essentially using it as an access point/switch (not as a NAT router), hence its DHCP server and NAT routing functions don't play a role.

Keep in mind that every time you connect a new device/MAC address to the modem you will have to reboot it, so that it recognizes the new device. Why not reboot the modem?
The "DHCP Release/Renew" function in the router will only expire the IP leases of dynamic DHCP clients connected to it directly, it will not affect the connection between the router and the modem.

videobruce · Post by **videobruce** » Sun Dec 31, 2017 4:58 pm

This is only a Modem, I have NO control over it other than a power switch and the ability to change a unneeded password.
It's connected to the Router using the WAN port (I didn't know it could be done otherwise).

Nothing new was connected to the Modem, the Router was temporarily disconnected to protect the firmware flash from outside interference per 3rd party instructions. Then it was re-connected. Pretty straightforward.

The "DHCP Release/Renew" function in the router will only expire the IP leases of dynamic DHCP clients connected to it directly, it will not affect the connection between the router and the modem.

That was what I was looking for, exactly what that Router menu function did.

If I waiting after the reconnect, would the Router get it's Internet 'handshake' with the Modem back, or is the only way to get that is a Modem power cycle? At the time I couldn't access the Modem at all even thou the indicator lights showed communication.

Post by **Philip** » Sun Dec 31, 2017 5:05 pm

If the router does not reconnect after power-cycle, something may have changed during the firmware update. I would check to make sure the router is set correctly and reboot it again. If it doesn't connect to the internet, the next step would be to reboot the modem.

videobruce · Post by **videobruce** » Sun Dec 31, 2017 5:21 pm

There was no trouble after, again it is/was why is that necessary? Same Router before and after, why can't it re-establish itself as it was 5-10 minutes or so ago??

Post by **Philip** » Sun Dec 31, 2017 5:38 pm

It is usually necessary anytime you connect a new device to the modem, as determined by it's hardware MAC address. I am not sure what changed with the firmware update, but the modem may have perceived it as a different device.

videobruce · Post by **videobruce** » Sun Dec 31, 2017 6:09 pm

I fully understand if it was a 'new' device, but it's not, nor did the MAC change. The only change was a different 3rd party F/W.
Isn't the only thing the is any issue is a devices MAC address??

Post by **Philip** » Sun Dec 31, 2017 7:58 pm

My guess is the DHCP lease issued by the modem to the router was no longer valid, it was somehow considered to be leased to another device.

Post by **YeOldeStonecat** » Mon Jan 01, 2018 8:54 am

Sometimes unplugging the power from a modem for several minutes is required. If it's a modem that supports VoIP phones...you also have to remove the battery too..else plugging the electrical power doesn't power down the modem.

Last week we did a lot of maintenance and updates at a client with multiple locations. Same ISP at many of the locations....many updates or changes or swapping out of the firewalls went smoothly. But one location had a stubborn modem...reboots...nada...power cycle for 1 minute...nada. But power cycle for a couple of minutes and than a LOOOOONG 5 minute way for a slow boot up...finally it released the public IP. Same ISP and same modems and same firewalls at a bunch of the locations...most of which went quick and smoothly.

Sometimes you just a device that is slow to respond.