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在面向对象分析过程中，用概念模型来详细描述系统的问题域，用 (1) 来表示概念模型。 (2) 关系用于表示类与类、接口与接口之间的继承关系；在Java中，用 (3) 关键字来直接表示这种关系。

为了保障数据的存储和传输安全，需要对一些重要数据进行加密。由于对称密码算法 (1) ，因此特别适合对大量的数据进行加密。国际数据加密算法IDEA的密钥长度是 (2) 位。

某公司为便于远程员工在家里访问公司的一些数据，允许员工通过Internet访问公司的FTP服务器，如图3-4所示。为了能够方便地实现这一目标，决定在客户机与FTP服务器之间采用 (1) 协议，在传输层对数据进行加密。该协议是一个保证计算机通信安全的协议，客户机与服务器之间协商相互认可的密码发生在 (2) 。

在面向对象分析过程中，用概念模型来详细描述系统的问题域，用 (1) 来表示概念模型。 (2) 关系用于表示类与类、接口与接口之间的继承关系；在Java中，用 (3) 关键字来直接表示这种关系。

在实现文件系统时，为加快文件目录的检索速度，可利用“文件控制块分解法”。假设目录文件存放在磁盘上，每个盘块512字节。文件控制块占64字节，其中文件名占8字节，文件内部号和文件其他描述信息占56字节。设某一目录文件共有254个文件控制块，采用分解法前查找该目录文件的某一个文件控制块的平均访问磁盘次数为 (1) 。若目录文件分解前占用n个盘块，分解后改用m个盘块存放文件名和文件内部号部分，那么访问磁盘次数减少的条件是 (2) 。

为了保障数据的存储和传输安全，需要对一些重要数据进行加密。由于对称密码算法 (1) ，因此特别适合对大量的数据进行加密。国际数据加密算法IDEA的密钥长度是 (2) 位。

在面向对象分析过程中，用概念模型来详细描述系统的问题域，用 (1) 来表示概念模型。 (2) 关系用于表示类与类、接口与接口之间的继承关系；在Java中，用 (3) 关键字来直接表示这种关系。

某公司为便于远程员工在家里访问公司的一些数据，允许员工通过Internet访问公司的FTP服务器，如图3-4所示。为了能够方便地实现这一目标，决定在客户机与FTP服务器之间采用 (1) 协议，在传输层对数据进行加密。该协议是一个保证计算机通信安全的协议，客户机与服务器之间协商相互认可的密码发生在 (2) 。

Most IP layer-based proxy mechanisms, such as network address translation (NAT), only support uni-directional proxy, from the internal network to external network (the Internet). The proxy establishes the communication channel by (1) IP addresses. The IP addresses must be (2) . These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software (3) for each application that uses (4) channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with (5) IP addresses.

VRML is a (1) for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra (2) (z and time) to the presentation of documents. The applications of VRML are (3) , ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.
X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards (4) with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different (5) , from high-end visualizations to lightweight applications.

在实现文件系统时，为加快文件目录的检索速度，可利用“文件控制块分解法”。假设目录文件存放在磁盘上，每个盘块512字节。文件控制块占64字节，其中文件名占8字节，文件内部号和文件其他描述信息占56字节。设某一目录文件共有254个文件控制块，采用分解法前查找该目录文件的某一个文件控制块的平均访问磁盘次数为 (1) 。若目录文件分解前占用n个盘块，分解后改用m个盘块存放文件名和文件内部号部分，那么访问磁盘次数减少的条件是 (2) 。

在图3-1所示的系统中，员工小郭要上Internet需要三层交换机、防火墙、路由器3台设备（每种设备赋予相同的可靠度）均正常运行。若要求整个系统可靠度为0.98，那么每一种设备的可靠度至少为（）。

Most IP layer-based proxy mechanisms, such as network address translation (NAT), only support uni-directional proxy, from the internal network to external network (the Internet). The proxy establishes the communication channel by (1) IP addresses. The IP addresses must be (2) . These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software (3) for each application that uses (4) channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with (5) IP addresses.

VRML is a (1) for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra (2) (z and time) to the presentation of documents. The applications of VRML are (3) , ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.
X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards (4) with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different (5) , from high-end visualizations to lightweight applications.

Most IP layer-based proxy mechanisms, such as network address translation (NAT), only support uni-directional proxy, from the internal network to external network (the Internet). The proxy establishes the communication channel by (1) IP addresses. The IP addresses must be (2) . These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software (3) for each application that uses (4) channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with (5) IP addresses.

VRML is a (1) for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra (2) (z and time) to the presentation of documents. The applications of VRML are (3) , ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.
X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards (4) with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different (5) , from high-end visualizations to lightweight applications.

Most IP layer-based proxy mechanisms, such as network address translation (NAT), only support uni-directional proxy, from the internal network to external network (the Internet). The proxy establishes the communication channel by (1) IP addresses. The IP addresses must be (2) . These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software (3) for each application that uses (4) channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with (5) IP addresses.

VRML is a (1) for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra (2) (z and time) to the presentation of documents. The applications of VRML are (3) , ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.
X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards (4) with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different (5) , from high-end visualizations to lightweight applications.

Most IP layer-based proxy mechanisms, such as network address translation (NAT), only support uni-directional proxy, from the internal network to external network (the Internet). The proxy establishes the communication channel by (1) IP addresses. The IP addresses must be (2) . These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software (3) for each application that uses (4) channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with (5) IP addresses.

VRML is a (1) for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra (2) (z and time) to the presentation of documents. The applications of VRML are (3) , ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.
X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards (4) with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different (5) , from high-end visualizations to lightweight applications.

在计算机网络中从源到目标的信息流动的各个阶段都可能受到安全威胁，如果图3-2左边所示为信息流动的正常情况，那么图中右边所示则为信息流动受到危害的情况，该危害属于（）。

某公司局域网防火墙由包过滤路由器R和应用网关F组成，如图3-3所示，下面描述错误的是（）。

图3-5是虚拟主机示意图，图中“N”部分不可能是（）。