I found this which applies to my modem but I struggle to understand if a) its the reason and will help 2) how to implement iii) sequences
Netgear C6300BD
Guide to Setup Static Routes on Telstra Gateway Max C6300DB
Purpose
This guide will describe how to workaround a setup for static routes where destination IP addresses are in the same subnet as the gateway itself.
example
If you want to firewall your network, run your own website filter, block connections between particular devices, run bandwidth management, throttling, QoS or other reason.
You end up with network traffic that flows along this path
/<-> Normal Devices
Internet <-> C6300BD -> Your Static Gateway -> Filtered Devices
\<-----optional bypass return+-----</
Note: all devices are still physically connected directly to the C6300DB and can be either wired or wireless, only the data flow direction is altered. For best results,
your static gateway should be wired.
Reasoning
The reason for this guide is the poor implementation of Static Routes on the C6300DB. On any other router or operating system this is trivial. Simply set the static route as follows:
Destination: <device ip or first ip in block range>
Netmask: <appropriate netmask for ip or range>
Gateway: <your static gateway>
However on the Gateway Max, priority is given to the automatically determined routes and therefore the routing table doesn't follow the rules for longest prefix match for static routes in the same subnet as the router. Therefore it ignores the static route if the device is directly contactable, this behaviour is what we are working around.
Workaround Instructions
Strategy
We split the standard 192.168.0.0/24 subnet into two /25 subnets, one for normal devices and one for filtered. With a device that is linking the two, your static gateway.
This is the general strategy but you can tweak or modify the size of the subnets as you see fit. You can also choose to have a direct return path to the router so traffic is only filtered in one direction. Otherwise, the return path will be through your static gateway. This will reduce the load if your static gateway only has one ethernet port.
Steps
(optional) Factory reset your device by holding the reset switch at the back of the modem for more than 10 seconds, also note the device must not be in bridge mode, and obviously you wouldn't want it that way if you are following this guide.
DNS
- login to the Gateway Max
- click 'Advanced View'
- click 'Broadband Connection'
copy the IPs of the Primary and Secondary DNS servers
Static Route
- log into your Gateway Max, accessible from http://192.168.0.1, default is 'admin' and 'password'
- click 'Advanced View'
- click 'Games and Services' then 'Static Routes'
- click to add a route
Destination: 192.168.0.128
Netmask: 255.255.255.128
Gateway: 192.168.0.2
- click 'Apply' to add it to the table and then 'Apply' to submit the form
- click 'Local Area Network'
- alter the subnet, starting and ending ips
Subnet Mask: 255.255.255.128
Starting IP: 192.168.0.3
Ending IP: 192.168.0.126
- click 'Apply' the router will reboot
- now configure Your Static Gateway with
IP: 192.168.0.2
Netmask: 255.255.255.0
Gateway: 192.168.0.1
DNS: <dns instruction above> or your choice
- then configure your filtered devices with
IP: 192.168.0.129 – 192.168.0.254
Netmask: 255.255.255.0
Gateway: 192.168.0.2 or optionally bypass for a direct return with 192.168.0.1+
DNS: <dns instruction above> or your choice
Alternatives
You could disable the built in DHCP server and run your own instead of statically configuring IP addresses for each filtered device. Configure your DHCP server with the following:
Normal Device IPs: 192.168.0.3 – 192.168.0.126
Filtered Device IPs: 192.168.0.129 – 192.168.0.254
Netmask: 255.255.255.0
Gateway: 192.168.0.1 for direct, or, 192.168.0.2 for two way filtering, your choice, you could even split for normal or filtered ip ranges
DNS: <dns instruction above> or your choice
Another alternative is to place the Gateway Max router in bridge mode and purchase another router with a proper static route implementation and connect your network from there.
Disclaimer: use at your own risk, for the most part these instructions will produce the desired results on your network, but I take no responsibility for any loss in productivity if you mess up your network. I do offer this advice, the factory reset switch is on the back press and hold it for more than 10 seconds.
+technically, the optional bypass return, shouldn't work due to the subnet differences between the gateway max and filtered devices, but lets face it, this modem is buggy and it does work. Though your filtered devices may not be able to connect to the normal devices. If that is an issue don't bypass, simply set
Your Static Gateway (192.168.0.2) as the default gateway for the filtered devices.