Some ISP has given us one pool (10.X.X.X/28) of IP for broadband/internet leased line and I have some free ip that is not using then can I use that free public IP in other public network ? If yes then how? If no then why ?Please help?
It is possible, however, that each of the addresses you have been allocated does correspond to a public IP with static routing (I hope this is the case), but if this is the case, then no, you cannot use the public IP address in any other network. There are both technical and fiduciary reasons for this.
On the technical side, for it to be accessible, the other network would have to announce the block as it's own (at it's data centre, no less, not on your PC), and it can't do that without messing up the original ISPs announcements (depending on which provider is larger).
The financial reason would be that, well, ISPs pay for the privilege of IP address space - if another ISP started announcing my IP addresses as it's own without my permission or compensation, I'd have reason to be a bit annoyed.
It has happened that ISPs have accidentally announced other ISPs address space as their own, like a few years ago when Pakistan tried to block Youtube, it ended up causing Youtube to become completely unavailable because anyone typing in youtube.com would be directed to Youtube.com's IP address BUT half the world thought Youtube's IP address belonged to PCTL or whoever it was so it borked the whole situation quite badly because PCTL couldn't cope and the guys at PCTL's upstream (PCCW in Hong Kong) didn't act quick enough to filter out the incorrect announcements, resulting in intermittent availability of Youtube for about 2 days.
So to cut a long story short, no, you can't use one of those "free public IPs" on another network (but you can use 10.x.x.x on any LAN you like, assuming that LAN is using that range, of course... some might use 192.168.x.x or 172.16.x.x).