Router doesn't work after move

Question

A friend moved to new house in the same city and was experiencing some very strange behavior when trying to setup his internet. Charter/Spectrum came and installed the modem and they said everything working when they left. However, when he plugged in his router, he had no internet. His router [Amplifi Instant] was working great at his old house mere hours ago.

I had him factory reset the router and go through the setup process again but the same result occurred. He could get internet by plugging a laptop directly into the Modem worked or using an old Linksys router.

I suspected that ISP [Charter/Spectrum] was did like that the router was used recently at a different location so I told to wait 2 days and try his Amplifi Instant Router [which he preferred since it provided a faster wireless connection]. He emailed me back today and said everything is working!

Googling didn't provide any helpful advice while I was troubleshooting so I am hoping this will save someone hours of headbanging.

TL;DR: After moving homes (and using same ISP) either:

1. wait 24 hours before plugging in your router you used in your old house
2. Clone the MAC address on router to something different that was being used at the old location

I assumed the modem's MAC address being used on all your packets being sent to out on the internet because it’s the last device before getting out to the internet. However, thinking about it further I could imagine some business having multiple WAN IP addresses being service by one modem and therefore would need unique MAC address foreach WAN IP that it would get from the router. Am I correct in this thinking? If so how does either the ISP or customer connect to modem for configuring since it because both the IP address and MAC address belong to the router?

Answer 1

I assumed the modem's MAC address being used on all your packets being sent to out on the internet because it’s the last device before getting out to the internet.

If the modem also acts as a router, then yes. (Routers always use their own MAC address, that way they can connect networks with different MAC addressing formats.)

There could be other reasons as well.

However, thinking about it further I could imagine some business having multiple WAN IP addresses being service by one modem and therefore would need unique MAC address foreach WAN IP that it would get from the router.

No, not necessarily. A single interface with one MAC address can have as many IP addresses as it needs to.

(Alternatively, a large business might have a dedicated IP address range routed to its network, in which case only one address is assigned to the router itself – while the dedicated range is forwarded "through" it.)

If so how does either the ISP or customer connect to modem for configuring since it because both the IP address and MAC address belong to the router?

The modem might actually have its own MAC address and obtain its own IP address on the same WAN (e.g. via DHCP). Being a bridge does not actually prevent the device from also being a host.

The WAN line might carry several different networks on their own VLANs – one for Internet access, one for management from ISP, etc. In this case, the modem might use DHCP to obtain an address in the "management" VLAN.

The modem might have a statically configured "local" address, such as 192.168.0.1, that is physically on the WAN line (completely unrelated to the customer's LAN) but really only accessible to directly connected devices. It's kind of messy but common for devices that don't really need much management.