多项选择题 Two devices on an Ethernet segment sent frames at the same time causing a collision. Which two statements are true?（）

单项选择题 You have been asked to use a 23-bit network mask to segment the network block 209.18.12.0, How many usable host address will there e per subnet?（）

单项选择题 You want to log into the router and make a configuration change.After loggin in as the root user, what is the next command you must enter?（）