The following directions are for setting up a segregated or 'guest' wireless network under libreCMC 6.x. Devices on your regular wireless or hardware LAN will not be able to communicate with those on this guest network.
This has been vetted under libreCMC 6.1 and should work with at least any router running the 6.x series version of libreCMC:
1. Open a web browser and log in @ https://192.168.10.1 or another IP if you've changed the IP for your router or otherwise received another configuration.
2. Go to Network > Interfaces.
3. Click Add new interface...
4. In the Name box enter guest.
5. In the Protocol drop down box select Static address.
6. Device drop down box can be left unspecified.
7. Click the Create interface... button.
8. In the IPv4 address box enter 192.168.12.1
9. In the IPv4 netmask box select 255.255.255.0
10. Click the Advanced Settings tab.
11. In the Use custom DNS servers enter 8.8.8.8 or another DNS server you'd like to utilize.
12. Click the + button.
13. Then 1.1.1.1 or another DNS server you'd like to utilize.
14. Click the + button again.
15. Under the Firewall Settings tab enter guest into the --custom-- field from the Create / Assign firewall-zone drop down box and hit the enter key to save.
16. Click the DHCP Server tab.
17. Click the Set up DHCP Server button.
18. Click the Save button and then the Save & Apply button.
19. Go to Network > Wireless.
20. Click the Add button.
21. In the ESSID field give the access point a name like guest.
22. In the Network drop down select the guest network.
23. Click the Wireless Security tab.
24. From the Encryption drop down box select WPA2-PSK (strong security) option.
25. In the Key box enter a password. This is what you will give out to guest users of your your guest network.
26. Click the Advanced Settings tab.
27. Check the box that says Isolate Clients.
Note: The Isolate Clients feature doesn't appear to be working in 6.1. The wireless guest wireless network will however be segregated from your regular wireless network and the hardwired LAN network.
28. Click the Save button, and then the Save & Apply button.
29. Go to Network > Firewall.
30. Click the Edit button for the guest zone.
31. In the Allow forward to destination zones: select the WAN network.
32. Click the Save button, then the Save & Apply button.
33. Go to the Traffic Rules tab.
34. Click the Add button.
35. Enter Guest-DHCP into the Name box.
36. Uncheck TCP in the Protocol box.
37. Select guest for the Source zone box.
38. Select Device (input) for the Destination zone box.
39. Enter 67 into the Destination port box.
40. Click the Save button.
41. Click the Add button.
42. Enter Guest-DNS into the Name box.
43. Select guest from the Source zon box.
44. Select Device (input) from the Destination zone box.
45. Enter 53 into the Destination port box.
46. Click the Save button, then the Save & Apply button.
47. Go to Network > Interfaces.
48. Click the Edit button for the guest network.
49. Click on the DHCP Server tab.
50. Change the Limit to 130.
51. Click the Save button, then the Save & Apply button.
You should now be able to connect to the guest network and surf the web. You will not be able to ping devices on the regular LAN.