in this setup we’ll treat goal by goal :

first so the host C and D communicates with each other, we have the internet destination is 167.168.229.0/26 so that means that host C and D should should be within that subnet , we know the mask of D so we can figure out the the ip of D1 and the ip and mask of R23 easily (eg. ip_D1: 167.168.229.2, ip_R23: 167.168.229.1, mask_R23: 225.255.255.240) and ofc the routes id D default destination and 167.168.229.1 as gateway , same thing for the host C (eg.

ip_C1: 167.168.229.19, ip_R22: 167.168.229.18, mask_R22: 225.255.255.240) and routes default destination and 167.168.229.18 as gateway.

now that we connected C with D we need to connect them to internet so we need to complete the connection of the two routers R21 and R13 ; we have the gateway of the router R2 so we know the ip of the R13 which is 167.168.229 so we need to choose a mask and based on it determine the ip of R21 bcs they would have the same mask. for the routing table of the router R1 we need to fill the destination by the subnet of our hosts which is 167.168.229.0/27 and the gateway by the ip of the router R21. and finally we fill the gateway of Internet routes by the ip of R12