Network setup - is this an ip problem (ethernet, not wireless)?

I'm setting up an office network (or rather appending 3 computers to a network which is already a domain but I don't know how to join the domain).

Anyway I set up the three ip addresses manually as 192.168.10.xxx and a subnet of FF.FF.FF.00.

At first the computers found the printer and installed drivers however today no computer (old ones or new ones) could locate the printer (HP3015P).

Looking at the printer, it's network address is 192.168.1.19 - is the reason it can't be found that the third byte is 1 rather than 10 or is there a more probable reason? When all the computers are on, looking at network connections shows two networks (which I can understand) - can both of them access the printer?

(I removed the other computers leaving the three new computers and the printer / modem as the only connected items and still couldn't connect to the printer - all three computers can access the internet)