Newbie here, regarding VLSM. How to configure routers with 2 switches.

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My main focus right is on the Ipoh Subnet. It has 60 hosts, split into two departments, one with 20 and the other with 40 hosts. I don't know how to configure the router. From my understanding, I am only allowed to use addresses 192.168.1.129 to 192.168.1.191, right?

Initially, I tried assigning the first two usable IP addresses to both of the interfaces linking it to the switches, both with the subnet  mask 255.255.255.192. It worked for the first interface, but for the second interface, it showed an error saying that it overlaps with the first interface.

Then, I tried the one as shown in the picture. I "split" the subnet address to 192.128.1.129/27 and 192.168.1.160/26. I chose .160 for my second interface because 128 + 32 = 160. But I think that it is not enough for 40 hosts because 191 - 160 = 31. I now have no idea what I'm doing. I just want to assign IP addresses to the two interfaces so I can continue with DHCP for the PCs.

Any help is highly appreciated.

[chrihussey]

Hello, 

I don't think it's possible to subnet the 192.168.1.128/26 to accommodate the two separate networks for the number of hosts specified.  At least not by using standard subnetting practices. If there is a lesson to be learned by this it would be to make sure you allocated enough IPs so that this isn't an issue. 

Anyhow, as long as you're being tasked with trying to figure this out perhaps IRB (Integrated Routing and Bridging) would be a possible solution. Basically keeping it a single /26 subnet, make the ethernet interfaces part of the same bridge group and then create a BVI (Bridged Virtual Interface) for the L3 interface.

https://www.cisco.com/c/en/us/support/docs/lan-switching/integrated-routing-bridging-irb/17054-741-10.html

Hope this helps.