When you have two router connected back to back all that we require is 2 IP on that segment.
so /30 would be the mask to use without wasting much IP since IP's are very costly.
/30 would provide you 4 address out of which 1 is used for network and 1 for broadcast and 2 valid IP's which can be assigned to the host. so when you use /30 for a back to back connection technicaly we aren't wasting any IP's
Technically we can use /31 as well for point to point link as the point to point link are serial links which does not nee network address (as only one device is connected at the other end and it knows where to forward) and serial are typically NBMA so broadcast address is not applicable.
In general I would advise designing the network so that networks/subnets within an area are in a common range (WAN and LAN together). This would be particularly important with OSPF where summarization can only be done on Area Border Routers. But it would also be helpful in protocols such as EIGRP since it would make it easier to summarize routes between areas.
While it might seem desirable from a management perspective to put WAN into one range and LAN into a different range so that you could identify the function of the network more easily, I believe that the advantage of being able to summarize routes should also be considered (and that the ability to summarize becomes more important as the network gets larger).
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