How to Calculate If Two Subnets Conflict

Multiple routers can be painstaking to configure if you don't set them correctly.

As a rule of thumb, when you are operating with multiple routers, you should not allow them to control the same subnet if you plan to let both assign Internet Protocols (IPs) to computers. This might cause many IP conflicts, especially if the computers do not have a static IP on the Local Area Network (LAN). Keeping track of how your packets are routed can really prevent a headache in configuring a network.

Things You'll Need

• Routers with operating manuals

Instructions

1. Read your routers' operating manuals beforehand and look for how to navigate to the place where you can configure Dynamic Host Configuration Protocol (DHCP) settings and the LAN IP of the routers, and find out the administrator account name, password and IP address for each router.

2. Open a new browser window or tab for each router you want to configure and write each individual IP address into each window or tab. Log in when you are prompted to do so.

3. Navigate to your DHCP settings and check out the range of IPs that the router can configure. There is a starting and an ending IP address.

4. Compare the ranges to make sure they are not on the same subnet. A subnet, in this context, refers to the first three numbers of an IP, starting from the left of the written sequence. Each IP is four numbers. If two routers are in control of "192.168.1.2 to 192.168.1.254," or one router is in control of "192.168.1.2 to 192.168.1.25" and the other is in control of "192.168.1.3 to 192.168.1.49," they are conflicting.

5. Change one of the numbers in the IP sequence to avoid conflict (i.e., change one of the router DHCP ranges to "192.168.2.2 to 192.168.2.254"). It is always a good idea to put "2" as the last number in the starting IP of your DHCP range so that your router can occupy the IP of "xxx.xxx.xxx.1" with "x" being a number in its subnet.

6. Go to LAN settings on the configuration pages. On all the routers you changed the DHCP range of, change the router IP to one that is in the same subnet they control but not inside the range.

Tips & Warnings

• Even if routers do not control "touching" ranges, you should still change the subnet they control to make things more distinguishable.

• You can set up a router to operate on the same subnet as another router if you can configure it not to act as a DHCP server and change its IP to something out of the other routers' DHCP ranges. This makes it "tag along" with the other router on that subnet and is useful if you want to chain the non-DHCP router to the one that is controlling IP assignment.

References

• Softpedia: What Is DHCP?