See Alphabetarian, you are to concerned with the number factor but when you tell the router you want to run in a class B network by assigning the subnet mask 255.255.0.0 you have done so, now this doesnt say you cant use the numbers 10.155.5.23 as the IP address to the Host. The hole number thing is just a rule to let you know how many nodes can run on that particular class address. You can do some research by picking up the book Advanced Network + by CompTIA.