How to subnet ...I just cram its formulas .....???

networks are divided in to five class (A,B,C,D,E). each class is identified by mask. eg. class A mask is 255.0.0.0 and class ip range is 0-126.0.0.0. here 0-126 are network id and rest is address bid. so if you have ip addres 10.1.1.4 then the network id is 10.0.0.0 and mask is 255.0.0.0

now if you use mask 255.128.0.0 with the same ip address then it means that you have subnetted the network further using class b mask and it is called as subnet mask.

so you can have many different network under main network 10.0.0.0.

well i tried to make it simple but you need to lear it carefully. just read ccna coureware and you will get it clear.

If you are confusing with calculation of sunetmasks you can use online subnetmask calculators.If you just give the ip address it gives every thing.
example site
http:// ccna.exampointers.com/sunet.phtml