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I have Comcast service with 5 static IP addresses, using a Cisco DPC3939B as the cable modem. In addition to the 5 machines with static IP addresses, there are additional devices that get their IP addresses dynamically from the DPC3939B using DHCP.

Everything works as expected, except that we don't have connectivity from clients on the DHCP devices to servers on our static-IP machines.

Here are some details:

The static IP addresses assigned are A.B.C.209, A.B.C.210, A.B.C.211, A.B.C.212, and A.B.C.213. The Cisco gateway's IP address is A.B.C.214.

The devices using DHCP are assigned addresses of the form 10.1.10.x. The Cisco gateway's IP address on the DHCP LAN is 10.1.10.1.

So, for example, consider Machine 1 that uses DHCP and has IP address 10.1.10.19, and Machine 2 (running a server) that has static IP address A.B.C.212. Machine 1 cannot connect to a web server or ssh server on Machine 2.

(In contrast, connectivity to and from remote sites works just as I expected: both Machine 1 and Machine 2 can connect to remote servers on the Internet, and remote machines on the Internet can connect to Machine 2. Connectivity from one static-IP machine to another static-IP machine also works.)

I tried temporarily turning off all firewalls, but that didn't correct things, so it doesn't appear to be a firewall issue.

It doesn't make any difference if the machines are connecting over wifi or a hard-wired Ethernet connection; the results are the same.

Occasionally the connection appears to start to work, but very slowly (too slow to be usable at all) and only for a short while.

If I ping Machine 2 from Machine 1, sometimes all packets fail to get through, but sometimes some of the packets show results like:

Request timeout for icmp_seq 0
92 bytes from 10.1.10.1: Redirect Host(New addr: A.B.C.212)
Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 91a8   0 0000  3f  01 8ad6 10.1.10.19  A.B.C.212

Comcast tech support thought that maybe I needed to set up a static routing table on the Cisco gateway, but that hasn't worked either (but maybe I just don't know how to set it up correctly).

Can anybody suggest what I could try so that our DHCP-based machines can talk to our machines with static IP addresses?

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    Can you include a network drawing? At the moment it seems obvious that if both are in the same layer-2 network but using different IP ranges, they can’t communicate with each other. – Tommiie Jan 11 '19 at 6:28
  • There's not a complicated topology. All the devices just hang off the cable modem, either using their preassigned static IP address or a DHCP-generated address. I had thought that since the cable modem is multi-homed (being on both networks), there should be a way for the DHCP-based devices to talk to our static IP devices in the same way the DHCP-based devices talk to remote servers on the Internet. – Mitchell Spector Jan 11 '19 at 6:31
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When the A.B.C.212/29 devices (public IPs??) are located in the same L2 segment as the 10.1.10.0/24 devices you'd need a router with hairpinning support (routing the destination back through the interface the source is located on).

A better and more secure way to set up the network is to use different router interfaces for each subnet (which a DMZ zone for the public IPs) and route between them properly.

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  • Thanks for the reference to "hairpinning"; I wasn't famliiar with it, so I looked it up and it's exactly what I'm asking for. I'm stuck using the Cisco DPC3939B provided by Comcast, since they don't support static IP addresses on customer-supplied routers. If there's a way to do what I would like (with a DMZ or something else), could you elaborate? The requirements would be: (1) sticking with the Cisco DPC3939B, still having 5 publicly visible IP addresses, and providing connectivity to other devices via DHCP. – Mitchell Spector Jan 11 '19 at 7:33
  • I'm not familiar with the DPC3939B but basically, you don't put everything on one switch/VLAN but you use routed ports on the Cisco and connect them to different switches or VLANs. – Zac67 Jan 11 '19 at 17:55
  • I accepted the answer because it let me know that what I was trying to do wasn’t possible. (I found another way — run a DHCP server on one of the machines with a static IP address.) – Mitchell Spector Jan 12 '19 at 5:12

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