Furthermore, suppose the interfaces closer to the top and left of the diagram are the lowest-numbered ones. Let's use the first space for the Campus to Edge link, the second one for Campus to Elsewhere, and the third from Edge to Elsewhere.
You want to configure a serial interface for a WAN connection. 10.0.0.0/30, which is 10.0.0.0 - 10.0.0.3 with a subnet mask of 255.255.255.252 Configuring Serial Interfaces / Router Interfaces and Media from Cisco IOS Cookbook.Three example IP spaces that will work for this are as follows: For simplicity, I will use the lowest addresses in 10.0.0.0/16 in this example. You can start with the lowest addresses in your available range, or the highest, or you can use any pattern that makes sense. So, we will create three IP spaces, each with a 30-bit subnet mask. The smallest IP networks that have two usable IP addresses have subnet mask lengths of 30 bits, which is 255.255.255.252 when written out in dotted decimal form. We should not allocate an IP space that is larger than necessary, since that will take away from the addresses we can use elsewhere in the enterprise. Since each of the IP spaces we need to allocate has only two interfaces in it (one serial interface on one end, and another on the other), each IP space only needs two usable IP addresses. That means each red line on the diagram gets its own IP space. To assign IPs to interfaces, a good first step is to assign IP spaces (a range of IP addresses that can be used together) to connections. You need to assign IP addresses to the six serial interfaces that are connected as in the above assumption.The red lines on the diagram indicate point-to-point connections terminated by serial interfaces on the routers that are connected by red lines.None of the IP addresses in the network 10.0.0.0/16 are in use anywhere in the existing enterprise.This answer uses the following assumptions: