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2 Messages
Cisco DPC 3939B and /29 Static Block
Hi,
We ordered Comcast Business Class service with a /29 static IP block and received a Cisco DPC 3939B modem.
Is this modem capable of being configured for public DHCP for the /29 block? We have a Cisco router WAN interface that will be connecting to the modem and will be looking to receive its publicly routable IP address via DHCP from the modem. The Cisco will handle all of the private DHCP requirements on the LAN. I just need to have the Modem pass a public IP to the Cisco interface. We've been using the Netgear CG3000 and this public DHCP method has worked great for us but we were told by Comcast those were being phased out. Just need to ensure we can continue to meet our customer's public DHCP to the Cisco requirement with this new Cisco.
**note - we had to move to this method of ordering a /29 due to the strict customer DHCP requirement of the router behind the modem and being that the former IP Gateway supplied by Comcast with Business Class service could not truly be bridged. Is that still the case with this modem? If this modem can truly be bridged and pass along a public IP via DHCP to the Cisco behind it, we would be fine purchasing a Dymanic Public IP handoff from Comcast and save the IPv4 IPs.
Thanks in advance for any input here.
Beth_IRG
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2 Messages
10 years ago
Thanks for the response, train_wreck. The Cisco 881 router that will be connecting to the Cisco DPC modem does NOT need a static IP. It merely needs to receive a single, publicly routable IP via DHCP from the modem. We were told the previous modems we got from Comcast could not be truly bridged to do this. That's why we went the multi static/public DHCP server on the modem route.
So, Comcast support can put these DPCs in "true bridge mode" remotely? And, is that the only way to get it bridged? We have techs that we send out from our company to configure the modems after the Comcast Carrier Install to configure the public DHCP on the Netgears before connecting ot the customer's VPN router. Just wondering if our techs could call in to Comcast Support on the spot to get a modem bridged or if there are steps they can take themselves onsite to do this.
Thanks again for the quick reply!
Beth
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train_wreck
Gold Problem solver
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610 Messages
10 years ago
As far as i know, the DPC can't give you assigned static IPs via DHCP; they're static IPs, after all 😉 The Cisco is only capable of handing out private-space addresses, from a default pool of 10.1.10.0/24
What you could do is use 1-to-1 NAT, routing 1 of your public static IPs to a private 10.1.10.x address that your router pulls via DHCP. Optionally, if you don't need a static IP, you could just have Comcast put the DPC into bridge mode, where the DPC will just pass public IPs via DHCP. (You can actually connect up to 5 devices this way; Comcast allows BCI customers 5 dynamic public addresses as long as the rented modem is bridged, or you own your own modem)
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train_wreck
Gold Problem solver
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610 Messages
10 years ago
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VBSSP-RICH
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1.4K Messages
10 years ago
Hello Beth_IRG and welcome,
You can certainly use the DPC3939B (DPC) modem with staticIP address(es) and use the internal DCP DHCP server for minimum 189 dynamic IP addresses, simultaneously. In order for you to do this effectively your Firewall DHCP Server should be disabled to avoid any DHCP conflicts resulting in performance or packet loss. Another good reason for this configuration is to utilize your public and/or private wifi features that require the DPC internal DHCP server for operation. If you use the aforementioned network configuration you should have not problems to continue meeting your customer's public DHCP business needs.
Hope this helps you out.
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