subnet mask rather than default

as my concept about subnet mask is ...i-e...
192.168.1.1 ---> 255.255.255.0
it means that 192.168.1. are fixed and the 4th one is from 1 to 255 hostz ...
192.168.1.1---->255.255.0.0
it meanz that 192.168. are fixed and 3rd , 4th (both have) are from 1 to 255 hosts...
so on...
1sr lets me clear plz concept mine iz correct ?

so my Question is that rather than default mask if there is 255.255.128.0 or 255.255.0.128 ... so what does it mean?

and 1 thing noticed while giving ip addresses, subnet mask to router or switch interfaces as giving me message as bad subnet mask ... we can't specify any subnet mask of our choice like 17.17.17.1 255.255.255.128 (maybe it waz the case i got thiz message) (((???)))
: :

Please move this to the correct fourm:

as my concept about subnet mask is ...i-e...
192.168.1.1 ---> 255.255.255.0
it means that 192.168.1. are fixed and the 4th one is from 1 to 255 hostz ...
192.168.1.1---->255.255.0.0
it meanz that 192.168. are fixed and 3rd , 4th (both have) are from 1 to 255 hosts...
so on...
1sr lets me clear plz concept mine iz correct ?

so my Question is that rather than default mask if there is 255.255.128.0 or 255.255.0.128 ... so what does it mean?

and 1 thing noticed while giving ip addresses, subnet mask to router or switch interfaces as giving me message as bad subnet mask ... we can't specify any subnet mask of our choice like 17.17.17.1 255.255.255.128 (maybe it waz the case i got thiz message) (((???)))

In the meain time view this: www.youtube.com/view_play_list?p=EDF1B51D8E450C29

After viewing that you should understand why 255.255.0.128 cannot be a subnet mask.