Modem + Router different subsets or not?

[Etherealize 0]

Hi quick question. I have my modem bridged on  Lan IP 192.168.0.1 and my router is on 192.168.1.1 should they both be on the same subnet or is it correct to have them on a different subnet. I’ve googled around but with no definitive answer. Cheers.

[Netduma Fraser 12,128]

Yes that is correct and absolutely normal, nothing to worry about there!

[scrubosaurus 61]

On 2/13/2021 at 5:03 PM, Netduma Fraser said:

Yes that is correct and absolutely normal, nothing to worry about there!

but is there any problem if they are in the same subnet?

[Netduma Liam 460]

It won't let you set the LAN address to the same subnet as the WAN because you'll end up with duplicate DHCP servers etc and it will just break.

[Valvaris 3]

Hello @Etherealize

to explain how IP Address works we just keep it simple with your IP's

(XR-R2-R1) Router [Interface WAN] <---192.168.0.x /24 or 255.255.255.0--> Modem [Interface LAN 1]

Router WAN IP should be 192.168.0.2 and the Modem LAN IP is 192.168.0.1 depends on DHCP Settings from the Modem. The CIDR is 24 and refers on the bit's used for the subnet in that case 24 times a 1 or four times 8bit in Binary - Decimal 255 = 11111111 - So the Subnet is like 11111111.11111111.11111111.00000000 - This means that the first three octets are not allowed to change on the IP Address - 192.168.0.x - The only octet to allowed to change is the fourth one from 00000000 to 11111111 in Dec 0 to 255 and since we use a IPv4 Protocol we are not allowed to use the first and the last Address in that range. Example: 192.168.0.0 (Network Address) and 192.168.0.255 (Broadcast Address) - The ones we are allowed to use is 192.168.0.1 to 192.168.0.254 

That is were proper configuration can be of a advantage for IT-Security and Usability - Keep your ranges small - Example for the Router to Modem Network this is more then enough -> 192.168.0.2 for the XR and 192.168.0.1 for the Modem with the Subnet CIDR /30 - DEC 255.255.255.252 - Binary  11111111.11111111.11111111.111111 00 

Since we look at this in the Binary view it will look like:
11111111.11111111.11111111.111111 00 = 192.168.0.0 (Network Address)
11111111.11111111.11111111.111111 01 = 192.168.0.1 - Modem LAN IP
11111111.11111111.11111111.111111 10 = 192.168.0.2 - XR WAN IP
11111111.11111111.11111111.111111 11 = 192.168.0.3 (Broadcast Address)
255           .255            .255           .252 - Subnet Mask

You can setup this Static for both Interfaces XR WAN and Modem LAN - To get rid of Re-Addressing the WAN interface when the Lease is over from the DHCP Server from the Modem - because that has to go thru a procedure every time we love to call DORA -> Discover - Offer - Request - Acknowledge - With Static IP's there is no need for the device to process all this.

XR Example WAN IP Settings:
IP Address: 192.168.0.2
Subnet: 255.255.255.252
Gateway: 192.168.0.1
DNS1:  1.1.1.1 Cloudflare 
DNS2: 1.0.0.1 Cloudflare

Modem LAN IP Setup:
DHCP = Off
LAN IP also Device IP = 192.168.0.1
Subnet = 255.255.255.252

On the XR LAN Side you are totally fine and can keep it as is. ^^

If there are any question do not hesitate to ask.

Best regards
Val.

[Etherealize 0]

Thank you Val for the detailed response. My modem is connected to the R2 Wan port and uses a PPPOE connection.  Am I correct in thinking the IP address of the modem should be set to 192.168.0.1 Subnetmask 255.255.255.252 and then the R2 Lan set to 192.168.0.2 Subnetmask 255.255.255.252 with the dhcp range set to 192.168.0.3?  Hope that makes sense. Cheers. 

[Netduma Fraser 12,128]

You don't need to change anything, I'd absolutely recommend you leave it as it is. You won't receive any benefits from changing these settings.

[Valvaris 3]

On 5/3/2021 at 6:34 PM, Etherealize said:

Thank you Val for the detailed response. My modem is connected to the R2 Wan port and uses a PPPOE connection.  Am I correct in thinking the IP address of the modem should be set to 192.168.0.1 Subnetmask 255.255.255.252 and then the R2 Lan set to 192.168.0.2 Subnetmask 255.255.255.252 with the dhcp range set to 192.168.0.3?  Hope that makes sense. Cheers. 

Question: Does your Modem do PPPOE or does the XR do PPPOE?

The reason why I ask is that if the XR does PPPOE for you and your Modem dials in too with PPPOE dependent on the ISP it could happen that you get double billed for Internet Dialup. 

OR

If the XR does Dialup only then there is no need to change any settings!

OR

If the Modem only Dials Up and you have Private IPs on the Modem LAN and XR WAN Interface then the top settings apply.

@Netduma Fraser that is untrue - Only if there is a Private Range between Modem LAN and XR WAN 192.168.0.x - The DHCP Service will Lease after a certain time and can disrupt communication between the to devices effecting the whole Home-Network randomly. That is why Static IP is a good thing for Security and Stability between Modem LAN and XR WAN. Plus a Service can be turned off that is not needed DHCP on the Modem LAN.

Back to @Etherealize  

Your internal LAN on the XR can remain as is with the standard settings of 192.168.1.1-254 /24 255.255.255.0 since the Internal XR LAN is on a different Subnet / Broadcast Network. That is why a Router Exists to connect two different Subnets / Networks / Broadcast Network.

If you have any questions do not hesitate to ask.

Sincerely to both
Val.

P.S. Here a Network Diagram for better understanding if your setup is the same way:
 

[Newfie 665]

Double billed for a single line?? None of this really make sense. 
 

a modem is invisible, MPOa mode. A modem is an invisible transport layer device and your router handles the PPPoE. 
 

If you are really concerned with security I would suggest a different router that offers more security options. If you want a static ip and are relying on users connecting to an internal server you host I would strongly recommend SOHO based routers.

[Valvaris 3]

43 minutes ago, Newfie said:

Double billed for a single line?? None of this really make sense. 
 

a modem is invisible, MPOa mode. A modem is an invisible transport layer device and your router handles the PPPoE. 
 

If you are really concerned with security I would suggest a different router that offers more security options. If you want a static ip and are relying on users connecting to an internal server you host I would strongly recommend SOHO based routers.

That is why I mentioned it "could" some ISP's do that. (Other ISP's tolerate double DialUP)
On the MODEM part if it is transparent YES but if it is set to only Bridge the Ports with Internal (Private IP's) then NO - That is why I put the OR for different scenarios.  

Sincerely
Val.

 

[Newfie 665]

It’s not dialup? What country are you in?

Bridge is MPOa. 
 

how can you get billed twice, what isp does this?