in this setup we have the ip of R11 so we can create a mask for example /25 and based on it we create an ip for A1 bcs it will have the same mask (eg. 111.198.14.2) for the routes of host A we should do default destination and the ip of R11 as gateway

we have the ip of R12 so same thing we create a mask for it lets say also /25 and since R21 should be in the same subnet we should give it an ip in that range (eg. 111.198.14.253) for the routes of R1 (destiantion should be default and gateway should be the ip of the router R2 same thing for the routes of R2 the destination is default and the gateway the ip of router R1

for the interface R22 we simply creat ea subnet mask like 255.255.255.192 we give it and ip like 1.2.3.1 and C1 should have the same mask and an ip like 1.2.3.2, for its routes the destination should be default and the gateway is the ip of R22