判定下列IP地址中哪些是无效的，并说明其无效的原因。 131.255.255.18 127.21.19.109 220.103.256.56 240.9.12.12 192.5.91.255 129.9.255.254 10.255.255.254

（）命令用于查看网卡的MAC地址。

When the system upon which a transport entity is running fails and subsequently restarts,the(21) information of all active connections is lost. The affected connections become half-open, as the side that did not fail does not yet realize the problem.The still active side of a half-open connections can close the connection using a(22)timer. This timer measures the time the transport machine will continue to await an (23) of a transmitted segment after the segment has been retransmitted the maximum number of times.When the timer (24),the transport entity assumes that either the other transport entity or the intervening network has failed. As a result, the timer closes the connection, and signals an abnormal close to the TS user. In the event that a transport entity fails and quickly restarts, half-open connections can be terminated more quickly by the use of the RST segment, the failed side returns an RST i to every segment i that it receives. When the RST i reaches the other side, it must be checked for validity based on the (25) number i, as the RST could be in response to an old segment. If the reset is valid, the transport entity performs an abnormal termination.空白处（24）应选择（）

A、stops

B、restarts

C、expires

D、abandons

To compete in today's fast-paced competitive environment, organizations are increasingly allowing contractors, partners, visitors and guests to access their internal enterprise networks.These users may connect to the network through wired ports in conference rooms or offices, or via wireless access points. In allowing this open access for third parties, LANs become(6) .Third parties can introduce risk in a variety of ways from connecting with an infected laptop to unauthorized access of network resources to(7)activity. For many organizations, however,the operational complexity and costs to ensure safe third party network access have been prohibitive. Fifty-two percent of surveyed CISOs state that they currently use a moat and castle's security approach, and admit that defenses inside the perimeter are weak. Threats from internal users are also increasingly a cause for security concerns. Employees with malicious intent can launch (8) of service attacks or steal (9) information by snooping the network. As they access the corporate network, mobile and remote users inadvertently can infect the network with (10) and worms acquired from unprotected public networks. Hackers masquer-ading as internal users can take advantage of weak internal security to gain access to confidential information.空白处（8）应选择（）

A、denial

B、virtuous

C、complete

D、traverse

To compete in today's fast-paced competitive environment, organizations are increasingly allowing contractors, partners, visitors and guests to access their internal enterprise networks.These users may connect to the network through wired ports in conference rooms or offices, or via wireless access points. In allowing this open access for third parties, LANs become(6) .Third parties can introduce risk in a variety of ways from connecting with an infected laptop to unauthorized access of network resources to(7)activity. For many organizations, however,the operational complexity and costs to ensure safe third party network access have been prohibitive. Fifty-two percent of surveyed CISOs state that they currently use a moat and castle's security approach, and admit that defenses inside the perimeter are weak. Threats from internal users are also increasingly a cause for security concerns. Employees with malicious intent can launch (8) of service attacks or steal (9) information by snooping the network. As they access the corporate network, mobile and remote users inadvertently can infect the network with (10) and worms acquired from unprotected public networks. Hackers masquer-ading as internal users can take advantage of weak internal security to gain access to confidential information.空白处（9）应选择（）

A、reserved

B、confidential

C、complete

D、mysterious

最早出现的计算机网络是（）。