Explanation or URL for how ISP's handle customers DHCP registration

[joltdude]

Can anyone explain to me how ISP's do WAN DHCP.... Im referring to the nonstandard stuff like blocking you from registering a replacement router until the DHCP lease expires,is released, or the timeout period is reached... and how does it respond, does it just ignore the discovery request of the second device...

 

[Scooterit]

Not an expert on this stuff but isn't the registration done via the MAC address of the modem?

In my experience with Comcast I always have to verify the MAC address with them.

If its a modem only it doesn't do DHCP.
The larger boxes have Modem with a build in router as if you would have connected your own one to the modem only. The difference is that they can remotely control it if needed.

 

[stevech]

Can anyone explain to me how ISP's do WAN DHCP.... Im referring to the nonstandard stuff like blocking you from registering a replacement router until the DHCP lease expires,is released, or the timeout period is reached... and how does it respond, does it just ignore the discovery request of the second device...

varies from ISP to ISP. Mine, Time Warner Cable in So. Calif., has given me the same public IP address for many years at a time - until they make a major change. Has happened 2 or 3 times in 14 years.

DHCP between your router's WAN port and the ISP's modem hands out the IP address that your ISP assigned to your modem's serial number. As said above, this public IP address usually changes infrequently. I have heard that AT&T changes much more often than others.

You probably know that the DHCP on the LAN side of your router is your business to manage.

The MAC address and other modem ID info is used by the ISP to authenticate the modem after the modem is first registered by the installer or by you if you are doing BYOD. Unregistered modems are of course ignored.

 

[o2fst4uo]

Just putting my two cents out there:

In most countries, your ISP doesn't care what modem/router you use. I could unplug mine right now and plug a brand new one in and it will work.

 

[joltdude]

Just putting my two cents out there:

In most countries, your ISP doesn't care what modem/router you use. I could unplug mine right now and plug a brand new one in and it will work.

Not so much true of the US.. they associate mac addresses with your DHCP lease...

 

[azazel1024]

In a lot of cases this is because there is more between you and the DHCP server than just a wire.

For example, I have verizon FIOS, I have an optical network terminal between my router and verizon. I know it isn't my ONT handing out a lease. At a guess, since I also have a 24hr lock on the MAC for a new IP (without doing an IP release first), my best guess is that the DHCP sever recognizes the MAC of the ONT that the DHCP request passes through. If there is an IP address reservation with that MAC attached already, it refuses to grant another one. IP release, wipes the entry on the DHCP table, so when a new request comes in, it is happy to grant one as there isn't one attached to that MAC address from the terminal.

On the cable side of things, probably something similar. They'll use the MAC of some device downstream to limit the reservation. Maybe your cable modem, maybe the switch port on the local node that only you are attached to, or coax to fiber bridge (since most cable these days is only coax from the curb, the rest is fiber from the curb to the ISP, though I am sure still plenty of all coax deployments out there), or something like that.

 

[fistv]

They usually use the MAC address, my cable co allows up to 3 devices so it has never been an issue for me on the router side though it has been a couple of years since I had a switch on the modem with two routers on at the same time. On a couple of occasions when I have had to replace the cable modem of which they only allow one, I call the cable company NOC and politely ask them to change the MAC, give them the new MAC, usually takes them about 30 seconds and within 5 min the cable modem is up and running, if they don't add the mac it will never get an IP.