Access Modem webpage through UTM

Question

I cannot access my cable modem webpage from my network. Internet access works fine.

My network is set up as follows:

internet -> cable modem -> external NIC -> UTM -> internal NIC -> switch -> LAN

My ISP provider tells me that the IP address for the modem should be 192.168.100.1

The external interface gets an IP from the ISP via DHCP (Type: Ethernet, Dynamic IPv2 checked, Default GW checked).

The internal interface is 192.168.5.1/24.

When trying to access the modem webpage at 192.168.100.1, the Firewall Live Log shows default drops for external IP adresses going to my internet IP address.

I have tried adding in an Additional Address of 192.168.100.1/24 on the external interface, and then when I try and connect the Firewall Live Log shows default drops for my client (192.168.5.50) to 192.168.100.1

The only firewall rule I have in place is Internal Network -> Any -> Any. I have tried adding in various firewall rules, but the connection still gets blocked.

Anyone have any ideas?

Thanks

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RickAlderson · Answer

OK, I finally got this working. 
 In Interfaces -> Additional addresses add a new IP address to the external interface. My modem IP is 192.168.100.1 so I went with 192.168.100.2/24 
 Then you need to exempt 192.168.100.2 from masquerading. In Network Protection -> NAT -> NAT add a "no NAT" for 192.168.100.2 -> Any -> Any. Check automatic firewall rule. 
 Done 
 Hopefully it helps someone. 
 
 You might need to add a SNAT rule as well to translate to 192.168.100.2 if your modem only accepts logins from the same subnet.