单项选择题
In the implementation of VLSM techniques on a network using a single Class C IP address,which subnet mask is the most efficient for point-to-point serial links?（）
A. 255.255.255.240 B. 255.255.255.254 C. 255.255.255.252 D. 255.255.255.0 E. 255.255.255.248
单项选择题 Which IPv6 address is valid?（）
单项选择题 Which of the following protocols uses both TCP and UDP ports?（）
单项选择题 Which command enhances the 802.1D convergence time on ports that are connected to hosts?（）
