There's two ways of doing this:
- By specifying IP/Netmask
- By specifying IP range
The first one is less complicated but you may not be able to specify the exact range that you want. You can find info on netmasks here. Use an online netmask calculator for help (like this one). For example, if you specify something like 192.168.1.128/25, the rule will apply to addresses in the range of 192.168.1.128 - 192.168.1.254.
The second method will require a lot more work, but ultimately you will be able to specify an IP range more conveniently and precisely. The router uses iptables for packet filtering. iptables supports a parameter called match (specified as -m). It specifies a match to use, that is, an extension module that tests for a specific property - a range of IPs in your case, which can be specified like this: -m iprange. However, for this to work you would have to install a special iptables extension called iptables-mod-iprange. To do so, follow these steps:
- Login to the router via CLI or SSH (instructions can be seen here)
- Execute the following commands:
opkg install iptables-mod-iprange --force-depends
- Move the installed module to proper kernel directory. Use this command to do so:
mv /lib/modules/3.18.23/xt_iprange.ko /lib/modules/3.18.44/
- For this to take effect, either insert the module manually (with this command: insmod /lib/modules/3.18.44/xt_iprange.ko) or simply reboot the router and it will take care of it automatically.
- Now for the rule itself. Go to the router's WebUI, Network → Firewall → Custom Rules section, enter this rule and save it:
iptables -I FORWARD -m iprange --src-range 192.168.1.50-192.168.1.99 -j DROP
When you do this, the router will not forward any traffic coming from 192.168.1.50-192.168.1.99 to zones other than LAN. So in the end result, devices from the specified range will be able to communicate in LAN, but not anywhere else. If you need them to communicate in zones other than LAN (like VPN, for example), reply to this answer and I'll help you add exceptions that will allow this.
Anyway, good luck and I hope this helps.