After several questions about setting up your own home lab from my previous post Home Labs: Getting the most out of one, the main problem is that their Sophos UTM is getting a private IP address and not their public one from their provider. This is because their media changer, cable modem, is handing out DHCP addresses and is not set to bridged mode. The default setting for a residential cable internet connection is routed with or without NAT, or some manufactures call it the residential gateway function. This function means the cable modem is acting as the gateway between your router and the internet.
Arris is the most popular cable modem that is in use in both residential and commercial connections. Because of this I will be explaining how to change the setting on this brand, other brands are very similar and the overall process is about the same.
The first step is to log into the modem, generally the address is 192.168.0.1 but a quick check of the label should let you know for sure.
Next you will be prompted for a username and password. Many times it is admin for the username and password for the password, if that does not work check the label on the modem and see if it has a different password printed on it. If neither of these steps work you may need to contact your provider for support, some companies put different passwords to ensure the end user does not change it.
After you are logged in you need to navigate to the LAN setup tab. From here we will be changing two items: under DHCP Server Setting we will uncheck the Enable DHCP server
We will then scroll down to NAT and change the NAT Mode to Bridged.
Click on save to save the settings, or some units automatically come up with a dialog box so you can select OK. At this point the unit may let you know that it will be restarting in a dialog box; click OK and the unit will take several minutes to update and restart.
After the modem power cycles you should not be able to access the log in page. At this point your modem is working in bridged mode and you may have to click on renew in your Sophos UTM WAN link in order for it to get the new address.