多项选择题 Given the address 192.168.20.19/28, which of the following are valid host addresses on this subnet?（）

单项选择题 在ISDN网络中，U接口定义在（）之间。

多项选择题 A national retail chain needs to design an IP addressing scheme to support a nationwide network. The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per subnet. Working with only one Class B address, which of the following subnet masks will support an appropriate addressing scheme? （）