Tell us a bit about your network including whether it is all peer workgroups or domain and the operating systems involved.
Routers, switches, DNS servers etc.
Public or private address ranges?
Private address blocks being:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
and also
169.254.0.0 -169.254.255.255 (which is generally reserved for automatic IP addressing - http://www.duxcw.com/faq/network/autoip.htm
Could be as simple as a router not being configured to assign more than a few addresses.
We have 200 ip working but still we are not able to access the entire 200 at a time .We are able to work on some part and when we change ip on the system we are able to access but assign ip is not used. it come occasionaly in assighn IPs.

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