we've expanded our business to a nearby building with its own network and i am trying to join the two so that the workstations can see each other, but they continue to use their own internet access. Here is the layout:

NETWORK 1 (aka N1): 192.168.1.*/16 => Switch: CISCO SG200-28P ( => Firewall: WATCHGUARD T30 ( / DHCP server .100-.200) => INTERNET

NETWORK 2 (aka N2): 192.168.30.*/24 => Switch: DLINK DGS1210-28P ( => Firewall: FORTINET 60C ( / DHCP server .110-.254) => INTERNET

At this point, i have run a cat6 cable from one switch to the other. When i connect my laptop to N1, i get a DHCP response/IP address from the server on N2. I can then connect to N2, but still can't see N1 even though i am connected directly to that switch.

If i give my laptop a static IP address on the N1 network, it functions normally and i can see all the nodes on N1, but nothing on N2.

Conversely, connecting directly into N2 gives me access to N2 nodes, but no access to N1. As stated above, i have changed the netmask on the switch and router (and my laptop) of N2 to be /16, but when i go to change it on the router of N1, i get a "IP address is in same subnet as the others." even though no other interface is on any 192.168 subnet.

What i am hoping to achieve is that a computer on N1 can talk to any computer on N2 (i am currently not concerned about vice-versa), but each node on the respective networks use their respective routers/gateway to the internet and DHCP addressing.

Any help would be very much appreciated. Thanks in advance.

EDIT Quick network diagram w/ device models. Pretty straightforward, really: enter image description here

  • You really need to give us a network diagram, and the network device models and configurations. Edit your question to include that, and we can see what you need to do.
    – Ron Maupin
    Sep 20, 2016 at 20:19
  • The diagram is good, but what are the network device configurations?
    – Ron Maupin
    Sep 20, 2016 at 22:07
  • Please don't post your real firewall configurations onto the internet! Its like putting a neon sign up saying hack me. This video is a pretty good starting point for networking theory. youtube.com/watch?v=t9TmvFvYfWw
    – tech-sean
    Sep 21, 2016 at 0:21
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 15, 2017 at 3:35

2 Answers 2


Without more information, I can only give you general ideas.

First, never just connect two networks together via a layer-2 switch unless you set up a specific VLAN for this. You don't want to bridge separate layer-2 domains.

Next you will need to let each router know about the networks on the router, and how to get to it. This will be on a separate interface, either logical or physical. Routers learn about networks in three ways: directly connected network, statically configured network, or by a shared routing protocol.

Simply connecting a cable between the two networks is probably going to lead to big problems for the users.

If you provide the information I asked for in the comment, we can give you more specifics on how to achieve what you want.

  • Thanks for the response. Not sure what else you need, re: configuration. Both networks are on a single VLAN each. Both networks have mostly statically assigned workstations, IP cameras and access points. We have a handful of mobile devices over wifi which are all DHCP-assigned. We just need to be able to have the networks see each other. I had a thought that opening up the subnet mask and connecting the switches would do the trick, and i assume that work, but that'd put two DHCP servers on the network and add confusion. So i'm happy to join at the router level with some guidance. thx,
    – airyt
    Sep 20, 2016 at 22:11
  • Just copy the configurations from the devices, paste them in your question, and use the preformatted text button on them.
    – Ron Maupin
    Sep 20, 2016 at 22:12
  • apologies for being such a noob. how do i copy the configurations from the devices? For example, I can get a configuration report from the watchguard, but it's 23 pages long
    – airyt
    Sep 20, 2016 at 22:28
  • The pre-formatted text feature will put the configuration into a scroll box. You just copy it, paste it into your question, obfuscate passwords and public addresses, highlight the text, and press the Pre-formatted Text button: {}.
    – Ron Maupin
    Sep 20, 2016 at 22:32
  • Sorry Ron, i can find no such feature on any of the devices. best i can get you is the watchguard configuration in html (or 23-page PDF). The switches don't have that feature, nor does the Fortigate from what i can find... Again, is there something specific you are looking for? everything else about the overall configuration seems quite straightforward to me.
    – airyt
    Sep 20, 2016 at 22:48

Here is a high level answer:

You will connect your two firewalls together (hopefully, you have vacant ports. If not, you will need to buy additional routers). The firewalls will forward traffic from one network to the other and vice versa. The link between the firewalls will have its own IP subnet (, for example).

Next, you will configure static routes on the firewalls so that each firewall knows how to reach to other.

Finally, you will configure the firewall policy to allow traffic to flow from one network to the other.

  • Thanks Ron - yes, there are spare ports on each router. Your answer differs from the other solutions i have read about. The other solution is that i would assign an IP from the opposite network to the new interface on each router, then add the static routes. Your suggestion is to create an entirely new subnet and use 2 IPs on that subnet for the new interfaces on the routers and add the static routes. Is there a difference or preferred solution? I haven't been back on-site since i posted. Will visit again on Friday and want to have a cohesive plan... thx again.
    – airyt
    Sep 22, 2016 at 1:11

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