DHCP Server Always Assign Subnet Mask 255.255.0.0 How Do I Change That? (Tx_Rx)

Pardon me if this has been asked somewhere,I'm using TCP IP Stack v5.42.08 Device PIC24FJ256 + ENC28. When I enabled DHCP server then connect the board to my PC (directly to the ethernet port), it always assign my PC with IP address 169.254.0.100 and sub..