The network given is 251.12.10.110/28
so the host bits will be 32-28=4
thus the last 4 bits of the fourth octet is reserved for hosts .
251.12.10.01101110
these last four bits if changed to 0 we will get the first address of the network
i.e 251.12.10.01100000
and if changed to all 1 gives the last address and it is not assignable to any host.
i.e 251.12.10.01101111
as the first and last addresses are reserved .