You are the network administrator for Acme Corporation in San Francisco, with dozens of WAN links connecting to your corporate office. Right now your network is a classful network, which means that all hosts and router interfaces have the same subnet mask on each interface. You’ve read about classless routing where you can have different size masks, but don’t know what to use on your point-to-point WAN links. Is the 255.255.255.252 (/30) a helpful mask in this situation?

Yes, this is a very helpful mask in wide area networks.

If you use the 255.255.255.0 mask, then each network would have 254 hosts, but you only use two addresses with a WAN link! That is a waste of 252 hosts per subnet. If you use the 255.255.255.252 mask, then each subnet has only two hosts and you don’t waste precious addresses.
This is a really important subject, one that we’ll address in a lot more detail in the VLSM network design section later in this chapter.