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?（）

单项选择题Which IPv6 address is valid?（）

A. 2031:0:130F::9C0:876A:130B
B. 2001:0DB8:0000:130F:0000:0000:08GC:140B
C. 2001:0DB8:0:130H::87C:140B
D. 2031::130F::9C0:876A:130B

单项选择题Which of the following protocols uses both TCP and UDP ports?（）

A. SMTP
B. Telnet
C. FTP
D. DNS

单项选择题Which command enhances the 802.1D convergence time on ports that are connected to hosts?（）

A. spanning-tree backbonefast
B. spanning-tree uplinkfast
C. spanning-tree portfast
D. spanning-tree cost512

单项选择题Which IEEE standard protocol is initiated as a result of successful DTP completion in a switch over FastEthernet?（）

A. 802.3ad
B. 802.1w
C. 802.1Q
D. 802.1d

单项选择题Which command helps a network administrator to manage memory by displaying flash memory and NVRAM utilization?（）

A. show secure
B. show file systems
C. show flash
D. show version