Network A                                   Network  B

      PC1                PC2                     PC3                PC4         
(    (         (    (
       |                  |                       |                  |
    --------------------------                ----------------------------
                |                                           |
             Router A ---------------------------------  Router B
          (                                (

I have an internet setup like this. I can't change the IP address of the network A because I've so many devices connected to the network. I can pretty much change any settings on the router B. what should I do so that device on the both network can communicate with each other

  • 1
    Have you implemented static routing between network A and B? If not, please do. – Aulis Ronkainen May 10 at 18:59
  • I redrew your network diagram. Please double check it. – Appleoddity May 10 at 19:22
  • Questions - do you have (or are you likely to ever have) more then 250 devices on your network? Are you statically allocating IP addresses out of A [without using DHCP to do so].- by this I mean is there anything stopping you from changing the netmask of devices behind router A? – davidgo May 10 at 19:51
  • Ladtly, are Aand B connected via ethernet? – davidgo May 10 at 19:52

The following is the solution in it's simplest terms. Whether your equipment can support this or not, is another story.

  1. Disable NAT on Router B
  2. Create a static route on Router A:

Dest Network: Next Hop: <The WAN Interface IP of Router B>

  1. Disable the Firewall on Router B, or enable firewall rules to allow communication between both networks.
  • I believe static routes are supported by almost every router in existance. Don't quote me on that though =) – Aulis Ronkainen May 10 at 19:36

This solution is a little more work, but another alternative would be to merge the two networks into a larger subnet. This solution offers the advantage that Router B is no longer needed, and can be surplused to that "other project" you might have that needs a router.

           Network A                            Network  B

      PC1                PC2                PC3                PC4         
(    (    (    (
       |                  |                  |                  |
                                 Router A 

In the DHCP settings on Router A, change the DHCP pool to be, or netmask if your router prefers that notation. All hosts on Network B will need to renew their DHCP leases to regain connectivity. Network A hosts only need to change their netmask.

Also, if you have "too many devices" to renumber, you may want to look into DHCP reservations. "Too many devices" is one sign that your network is beginning to outpace your management tools. DHCP reservations makes it easy to have known, static IPs assigned to devices that need them, and still have the other network configuration components (netmask, default route, DNS resolvers, etc.) fluid and readily changeable.

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