The physical connection between Core 1 and D2 has 192.168.1.4/30 subnet as you set it up. So you just need to advertise it exactly as it is in OSPF. Same goes for all other subnets.
There is also another way, you could summarize a bunch of contiguous subnets into one OSPF command. like this:
network 192.168.1.0 0.0.0.15 area 0
But this is dependent on the network you have, some times you might not have contiguous networks. In your case, you had 4 contiguous point-to-point subnets that can be summarized by 192.168.1.0/28 (i.e 0.0.0.15). The first method is guaranteed to always work. Here is the file for the second method.
www.mediafire.com/?w9szbe24e03byc6
