I am trying to allocate 256 IP addresses to Subnet1, 128 IP addresses to Subnet2 and 128 addresses to Subnet3 using the IP address 120.130.10.0/23
What I have setup in GNS3 is a router connected to 3 ethernet switches, and each of these ethernet switches have a VPC connected for testing purposes. 
What I think should be done is assign 120.130.10.1-120.130.11.2 to Subnet1, 120.130.11.3-120.130.11.130 to Subnet2 and then since there isn't 128 IP's left, I should create a new network space 120.130.12.0/24 to allocate 120.130.12.1-120.130.12.128 to Subnet3.
Now I'm assuming that what I've said above works ? (beginner network knowledge)
But I run into an issue when I'm trying to apply this to GNS3 because I can't assign two subnets to 120.130.10.0/3 because I get the error "120.130.10.0 overlaps with FastEthernet0/1"
So I would like to know how to fix this problem when it comes to configuring the routers, or any other problems you can see with my logic as I'm new and any help will be appreciated
Best Answer
You cannot assign IP addresses to the subnets the way you describe. The number of addresses in a subnet must be a power of two, and that includes the unusable-for-hosts network and broadcast addresses. If you must have 256 hosts in one subnet, you will need to use a
/23network.Also, you cannot have overlapping subnets configured in a router, as you have discovered.
This answer will give you the basics on how to subnet correctly. You need to convert the addresses and masks to binary to do it correctly, and you will see how it works. In binary, it becomes obvious.