JDMils
Level 6: Bloodhound

Setup the Telstra Smart Modem Gen2 DHCP to use more than one subnet (IP range)

OK, so I have OCD, and I like everything setup nice & neat. So when I installed the new Smart Modem, I tried to setup the following DHCP scopes:

Modem: 10.0.0.1

Static IPs: 10.0.0.10 -> 10.0.0.49

DHCP: 10.0.1.1 -> 10.0.1.254

 

As you can see, I opted to use separate ranges for DHCP, one for static IPs and one for dynamic IPs. Here's how to set it up:

  1. Go to the Local Network box.
  2. Under DHCP Settings, enable DHCP Server, enter DHCP Start as 10.0.1.1 and DHCP End as 10.0.1.254.
  3. In the same box, under Global Information, change the Local Network Subnet to 255.255.254.0.
  4. Save the settings.

The import part of this setup is step #3, where you change the subnet mask. I won't go into the technicalities of how the subnet mask works, you've got Google for that, but the value 255.255.254.0 means: "I can access any IP address in the range 10.0.0.0 to 10.0.1.255".

That's it!

Oh, BTW, in the Local Network box, you can setup static leases, that is, give a device an IP based on its MAC address- you are NOT limited to the IP addresses in the DHCP range, even if the device is setup for DHCP. I've configured 2 rules for 2 notebooks which are setup for DHCP to be given an IP from my static IP scope! Pretty cool!

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4 REPLIES 4
JDMils
Level 6: Bloodhound

Re: Setup the Telstra Smart Modem Gen2 DHCP to use more than one subnet (IP range)

I took the DHCP setup one step further: I added another 2 subnets to my router's DHCP range, one per Guest SSID.

I setup the following DHCP ranges for each of the Guest SSIDs:

Guest1:

Local Device IP: 10.0.2.1

Local Network Subnet: 255.255.255.0

DHCP Start: 10.0.2.10

DHCP End: 10.0.2.250

 

Guest1_5Ghz:

Local Device IP: 10.0.3.1

Local Network Subnet: 255.255.255.0

DHCP Start: 10.0.3.10

DHCP End: 10.0.3.250

 

I tested this setup and it works as designed. All devices on the Guest SSIDs have internet access. One thing is that no devices on the main subnets (static subnet 10.0.0.x nor DHCP subnet 10.0.1.x) can access the devices on the Guest SSID. I suppose this is OK since the Guest SSIDs are only to provide internet access. However, If I wanted to allow the access to the Guest SSID devices, I would have to change the subnet mask for the router to 255.255.252.0 which would give the router access to the subnets 10.0.0.1 - 10.0.3.254.

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Mkrtich
Level 21: Augmented
Level 21: Augmented

Re: Setup the Telstra Smart Modem Gen2 DHCP to use more than one subnet (IP range)

Hi - congratulations on your research and findings. It appears you have discovered a way of segregating devices connected to the same modem, similar to a VLAN on a switch. Thank you for sharing this with Crowd Support.

 

With Guest WiFi Networks, I think devices that connect to their respective SSIDs are already prevented from accessing other devices on the home network and are only allowed to connect to the Internet. From memory , they use a hidden range 192.168.2.1 which can not be used as a substitute for 192.168.0.1 if you wished to allocate that former IP address to the main modem. 

JDMils
Level 6: Bloodhound

Re: Setup the Telstra Smart Modem Gen2 DHCP to use more than one subnet (IP range)

You are correct in saying that the devices on the Guest networks are segregated from the other networks- the devices connected to the Guest SSIDs cannot access the LAN network. I tried to setup the subnet 255.255.252.0 on the LAN network for the router IP, but that failed saying the router modem IP cannot be in the Guest network, and then tried setting up the subnet for the Guest network of 255.255.252.0 and the GUI errored with a message saying the Guest modem IP cannot be in the LAN network.

Looks like there's some input checking happening behind the GUI to stop the interaction between the Guest networks and the rest!

Note that the Guest networks don't have to be fixed to the default 192.168.2.x address. In my original example, I have changed both Guest networks to 10.0.2.x & 10.0.3.x ranges and it works as designed.

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Mkrtich
Level 21: Augmented
Level 21: Augmented

Re: Setup the Telstra Smart Modem Gen2 DHCP to use more than one subnet (IP range)

Thanks. Being inquisitive, I looked up what a 255:255:254:0 subnet mask does, so I think I made a wrong assumption about your original post with my comment about a pseudo VLAN effect. Seeing different ranges, I had incorrectly assumed that two networks were created and there was fence between them due to third octet being different.  It appears by doing this, it is possible to expand the number of users on your network to nearly double its size rather than have two networks that are segregated. 

 

Is this correct - so using 192.168.0.1 as an example, with a 255:255:254:0 subnet mask, the usable IP range is expanded from the original 192.168.0.1 - 192.168.0.254 (254 addresses) to 192.168.0.1 to 192.168.1.254 (508 addresses). 

 

Not sure if the modem's software limits the actual number of devices due to traffic expectations as the specifications say 254 devices (maybe because of the 255:255:255:0 subnet mask?) .

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