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user_83346a
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6 Messages

Saturday, January 31st, 2026

Modem in passthrough\Bridge will not give connected clients IP4

Hello, when I put my comcast business modem in pass through mode (we have our own firewall behind it) a laptop that trys to connect to the modem directly via internet or WIFI (Like a back door to bypass the firewall in a DR event) does not get an internal or external IP4 address.

 

My understanding is that DHCP is disabled. However, the client is still handed an ip6 address from what appears to be DHCPv6 although I can't say I'm familiar with ipv6 dhcp and stateless address configuration. With this they can browse websites that support ipv6 but fail on ip4 only websites like amazon.com for example.

1. How can a client that is connected directly to the Modem via wifi or ethernet still get an IP6 address even when I'm told DHCP is disabled in pass through mode.

2. How can I get ipv4  to a laptop when connected to WIFI or ethernet when the modem is in passthrough mode.

Thanks

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15 hours ago

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Comcast_Seth

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9 Messages

3 days ago

Hello user_83346a 👋 I appreciate you stopping by our forums for help, and you're in the right place!

When a gateway is set to Bridge Mode, it only allows one device to connect, which may prevent connected clients from obtaining an IPv4 address. This is because Bridge Mode disables certain network management features, including DHCP, which assigns IP addresses. To resolve this, ensure that the gateway is not in Bridge Mode if multiple devices need to connect. If Bridge Mode is necessary, consider connecting a separate router that can handle DHCP for the connected devices. Let me know if you have any questions on that 👍

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user_83346a

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6 Messages

20 hours ago

It must not disable DHCP completely as noted above machines that connect (wifi or lan port) to the modem in bridged mode get an ipv6 address and can browse IPv6 supported websites.

If i did get another router how to I truely disable DHCP including ipv6 dhcp on the bridged modem?

It sounds like if I don't need DHCP I can manually assign a static IP to clients connecting to the modem (Wi-Fi lan port) in bridged mode. If so, what is the IP range that they should be assigned to? The same as the  10.1.10.1 subnet? 

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Comcast_Richard

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52 Messages

Hi there, @user_83346a When a modem/router is correctly set to bridge mode (sometimes called "Modem Only" mode), it should automatically disable all internal routing, Wi-Fi, and DHCP services. Yes, if a modem/gateway is placed in  bridge mode, it acts primarily as a bridge (Layer 2 device) and turns off its internal DHCP server. When in this mode, you can manually assign static IP addresses to devices connected to it.-Richard

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19 hours ago

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user_83346a

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Ok, I see that there is a difference between pass through and bridged.

What is stated above while may be true for bridge mode is not true in pass through mode.

Pass through mode:

1. When the modem is in pass through mode WIFI still works.

2. When the modem is in pass through mode Clients that are connect to the LAN port on the back of the router or via the modems internal WIFI they get an IPv6 address from the modem so some type of ipv6 DHCP is enabled.

Why can I not get an IPv4 address assigned to a connected device when plugged into the modem just like it can get an IP6 address when in pass through mode? 

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18 hours ago

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Comcast_Marshante

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13 Messages

@user_83346a, With IPv4, most providers only allocate a single public IPv4 address to a customer. In pass-through mode, the modem is designed to forward a single IPv4 address to a single downstream device, typically your router. Once that address is assigned, the modem will not issue additional IPv4 addresses to other connected devices.

With IPv6, the modem isn’t assigning just one address. Instead, it receives an entire IPv6 prefix from the network and can distribute multiple IPv6 addresses simultaneously to different devices. That’s why devices can still obtain IPv6 connectivity even when IPv4 is limited to a single device.

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17 hours ago

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user_83346a

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@Comcast_Marshante​ So what is the answer then?

To buy another router and another static IP from comcast. Assing that new static IP from comcast  to the new router and connect that new router to the comcast modem? From the new router I can hand out ipv4 ips to connected clients to the router?  Also, Im not looking for additional public  IPs to be handed to a connecting client. Just internal ip4 so I can communicate out to the internet.

Or can I bypass buying another static and router and just manaully define 10.1.10.x IPs for clients who plug into the comcasts router. Does the 10.1.10.x network route out to the internet over the modem in pass through?

Thanks

(edited)

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17 hours ago

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Comcast_Richard

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52 Messages

Thank you, @user_83346a With residential services unfortunately you would not be able to buy another static IP, you may be able to if you had Comcast business services. , which allows them to communicate with the internet, without needing additional public IPs. Yes, a new router will absolutely handle assigning internal IPv4 addresses (e.g., 192.168.1.x or 10.0.0.x) to your connected clients via DHCP which allows them to communicate with the internet, without needing additional public IPs.-Richard

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17 hours ago

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