subnet calculator tool

Good calculator here for dividing up larger subnets into smaller ones
http://www.davidc.net/sites/default/subnets/subnets.html

A /16 breaks up into two /17's, a /17 breaks up into two /18's and so on

As you break up a /16 you get more networks but less hosts

mask	networks	hosts
/16	1	65534
/17	2	32766
/18	4	16382
/19	8	8190
/20	16	4096
/21	32	2046
/22	64	1022
/23	128	510
/24	256	254
/25	512	126
/26	1024	62
/27	2048	30
/28	4096	14
/29	8192	6
/30	16384	2