178.100.0.0 is generally regarded a class B address, so it will have 2^16 addresses.
For 2000 sub-nets, you will have to sub-divide the class B address into 2048 sub-nets or 2^11 sub-nets. This will leave you 2^5 addresses in each sub-net.
The mask will be 255.255.255.224.
Note: This address range is also used on the public Internet, and the address range is allocated to t-mobile.co.uk. If you use these addresses for a private sub-net they will not be able to work with t-mobile devices using the same address range. Also, you must make sure that you do not allow these addresses to reach the public Internet.
