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How to use static IP addresses

We recently purchased a block of 5 static IP addressess from Comcast.  I haven't done much more than home networking and so I'm at a bit of a loss about a couple of things, especially since the hardware pre-dates my arrival here.


Here's our setup: 

         DPC3939B Comcast Modem + WIFI                                    -- Router is used

                        |                                     |

         SonicWall TZ100                          |                                        -- mainly used for SSL VPN

                        |                                     |

         Ethernet Switch                            |

             | | | | | | |                                    / \

         About  15 devices                     5 - 10 Wireless devices


Comcast's website shows our IP range as from - with a gateway ip address of and the subnet mask of (as expected.)  On the Modem's website at under Gateway > Connection > WAN Status, it shows the "WAN IP Address (IPv4)" with the same value as the "Gateway IP Address" given on Comcast's site.  For "WAN Default Gateway Address" however I see  Is this correct?


The point where I'm really confused though is what to do with these static IP's in order to use them. I've read around the Comcast documents, some of the posts here, and a sampling of pages on the internet but nothing really makes sense. 


1) I don't want to buy hardware unless I really must.

2) I can't use the SonicWall as a Router since that would leave out devices connect via WIFI to the DPC3939B modem.


Could someone please give me some pointers? or maybe a few instantiated objects? ;-)


Finally, the Comcast documentation talks about a "True Static IP Subnet"--what do they mean?


Thank you for your time.



New Contributor

Re: How to use static IP addresses

I'm not sure how/if the Sonic firewall will affect things since I haven't used them. Ignoring that, to assign one of your static IPs to a device simply set that device to use a manual configuration and set it like this:


IP:  - where xxx is the IP you want to use.

Subnet Mask:  - The 248 designates that you have a /29 IP block (5 assignable addresses

Gateway: 96.XXX.XXX.198  - This is the gateway for your subnet


You will likely also have to set the DNS servers manually, to use Comcast's DNS servers use:


This assumes that your "Ethernet Switch" is really being a switch and not a NAT router. That means your modem is connected up to one of the regular ports and not a WAN port. 


Are you using this configuration now? If so then your switch (or the SonicWall) may be a NAT router and you will not be able to assign IPs to devices connected to it. To assign an IP to a device that device has to be directly connected to the modems network. Using a true switch to extend that network is OK but it must not be set up as a NAT router.


Obviously you don't have enough IPs to assign one to every machine as you list 15+ devices. You will have to set the machines you don't want to have static IPs to use DHCP meaning set them to acquire an address automatically. The modem will act as a NAT router and assign private internal IPs to the other devices.


Networking can get pretty complicated and there are multiple ways a network can be set up. It all depends on what your final objective is.


I set the modem to "Disable firewall for true static IP subnet" since I handle firewall duties furthur on down the line. 


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