This optional parameter defines the maximum number of ports that can be mapped to a class C or class B FramedGroupBaseAddress. The default is 255, which means that any address from 0 up to 255 in the 3rd or 4th octets will be permitted. It actually specifies the modulus for computing the 3rd and 4th octets of addresses calculated from FramedGroupBaseAddress. You might use this to limit the number of addresses used in each address block, or to prevent the allocation of the last address in a class C address block.