11. (简答题, 30分)

Given the network 199.141.27.0 with a subnet mask of 255.255.255.240, identify the valid host addresses from the following addresses and explain why others are invalid.

A. 199.141.27.33
B. 199.141.27.112
C. 199.141.27.119
D. 199.141.27.126
E. 199.141.27.175
F. 199.141.27.208



翻译：给定网络 199.141.27.0，子网掩码为 255.255.255.240，请从以下地址中识别有效的主机地址，并解释其他地址为什么无效。

A. 199.141.27.33
B. 199.141.27.112
C. 199.141.27.119
D. 199.141.27.126
E. 199.141.27.175
F. 199.141.27.208

标准英文答案

A,C,D are valid, others are invalid.The mask 255.255.255.240 indiates that this network uses 4 bits to express Host ID. Hence, the Host ID in "199.141.27.112" is (0000)B, the Host ID in "199.141.27.175" is (1111)B,the Host ID in "199.141.27.208" is (0000)B.And Host ID with whole 0 bits or 1 bits are not permitted.



翻译：A、C、D 是有效的，其他的是无效的。子网掩码 255.255.255.240 表示该网络使用 4 位来表示主机 ID。因此，“199.141.27.112”中的主机 ID 是 (0000)B，“199.141.27.175”中的主机 ID 是 (1111)B，“199.141.27.208”中的主机 ID 是 (0000)B。而全部为 0 或 1 的主机 ID 是不允许的。

12. (简答题, 10分) During a nomal IP packet forwarding at a router, which field(s) in the header should be updated? Why?

翻译：在路由器中正常的 IP 数据包转发过程中，应更新报头中的哪些字段？为什么？

标准英文答案

TTL; TTL is used to indicate the life time of the current packet. A decrement operation should be down when it goes through a router. Checksum; Since the TTL changed, the Checksum should be recalculated and refreshed.



翻译：TTL；TTL 用于指示当前数据包的生存时间。当数据包通过路由器时，应进行递减操作。校验和；由于 TTL 发生了变化，因此应重新计算并刷新校验和。

13. (简答题, 10分) Try to divide the network 172.16.20.0 with the mask 255.255.255.0 into 4 subnetworks and tell the available IP addresses and their corresponding masks for each subnet.

翻译：尝试将网络172.16.20.0，掩码为255.255.255.0，划分成4个子网，并告诉我每个子网的可用IP地址和对应的掩码。

标准答案