def resize(prefix_len)
			m128 = Mask128.new(prefix_len)
			return IPv6Net.new(self.network,m128)
		end
def resize(prefix_len)
			m128 = Mask128.new(prefix_len)
			return IPv6Net.new(self.network,m128)
		end
resize returns a copy of the network with an adjusted netmask.
 Throws ValidationError on invalid prefix_len.