The unknown-clients flag is used to tell dhcpd whether or not to dynamically assign addresses to unknown clients. Dynamic address assignment to unknown clients is allowed by default. An unknown client is simply a client that has no host declaration.

The use of this option is now deprecated. If you are trying to restrict access on your network to known clients, you should use deny unknown-clients; inside of your address pool, as described under the heading ALLOW AND DENY WITHIN POOL DECLARATIONS.

若配發的IP範圍是100~110,只有5台PC都有設定MAC,
那其他的也取得到IP.

所以要調整DHCP的IP範圍, 讓數量所設的MAC數量一樣,
這樣沒設定MAC的就取得不到IP.

不過既然已設定某個MAC固定配發IP, 這個IP應該會保留,不會發給其他USER才對.