So IP addresses 192.0.0.0 to 255.255.255.0 use a subnet mask = 255.255.255.0. Classless IP addressing means you can use any subnetmask you want, even assigning partial blocks. For example subnet 172.16.0.0 is a class B network. But you have two physical interfaces on your router that connect to switches with 5 VLANs.