Network Addres Translation(NAT) is an Internet standard that enbles a local-area network to use one set of IP addresses for internet traffic and another set of ( 1 ) IP addresses for external traffic. The main use of NAT is to limit the number of public IP addresses that an organization or company must use, for both economy and ( 2 ) purposes. NAT remaps an IP address space into another by modifying network address information in the ( 3 ) header of packets while they are in transit across a traffic routing device. It has become an essential tool in conserving global address space in the face of ( 4 ) address exhaustion. When a packet traverses outside the local network, NAT converts the private IP address to a public IP address. If NAT runs out of public addresses，the packets will be dropped and ( 5 )"host unreachable"packets will be sent.

（1） A.local     B.private     C.public     D.dynamic

（2） A.political     B.fairness     C.efficiency     D.security

（3） A.MAC     B.IP     C.TCP     D.UDP

（4） A.IPv4     B.IPv6     C.MAC     D.logical

（5） A.BGP     B.IGMP     C.ICMP     D.SNMP

參考答案：C、D、B、A、C

參考譯文：網(wǎng)絡(luò)地址轉(zhuǎn)換（network address translation，NAT）是一種因特網(wǎng)標(biāo)準(zhǔn)，它使局域網(wǎng)能夠?qū)⒁唤MIP地址轉(zhuǎn)換為用于內(nèi)部通信的IP地址，并將另一組公共的IP地址轉(zhuǎn)換為用于最終通信的IP地址。NAT的主要用途是確定應(yīng)用程序或公司必須使用的公共IP地址的數(shù)量，為了經(jīng)濟(jì)和安全目的，NAT通過在流量路由設(shè)備上處理數(shù)據(jù)包時(shí)修改數(shù)據(jù)包IP報(bào)頭中的網(wǎng)絡(luò)地址信息，將一個(gè)IP地址空間重新映射到另一個(gè)IP地址空間，在IPv4地址耗盡的情況下，NAT已成為保存全局地址空間的重要工具。當(dāng)數(shù)據(jù)包在本地網(wǎng)絡(luò)外傳輸時(shí)，NAT將專用IP地址轉(zhuǎn)換為公共IP地址。如果NAT用完公共地址，數(shù)據(jù)包將被丟棄，并且ICMP“主機(jī)不可訪問”數(shù)據(jù)包將被發(fā)送。

The TTL field was originally designed to hold a time stamp, which was decremented by each visited router. The datagram was ( 1 ) when the value became zero. However, for this scheme, all the machines must have synchronized clocks and must know how long it takes for a datagram to go from one machine to another. Today, this field is used mostly to control the ( 2 ) number of hops (routers) visited by the datagram. When a source host sends the datagram, it ( 3 ) a number in this field. Each router that processes the datagram decrements this number by 1. If this value, after being decremented, is zero, the router discards the datagram.This field is needed because routing tables in the Internet can become corrupted.A datagram may travel between two or more routers for a long time without ever getting delivered to the ( 4 ).This field limits the ( 5 ) of a datagram.

（1）A.received     B.discarded     C.rejected     D.tansferred

（2）A.maximum     B.minimum     C.exact     D.certain

（3）A.controls     B.transmits     C.stores     D.receives

（4）A.switch     B.router     C.source host     D.destination host

（5）A.lifetime     B.moving time     C.receiving time     D.transmitting time

參考答案：B、A、C、D、A

參考譯文：ttl字段最初被設(shè)計(jì)用來存放時(shí)間戳，每個(gè)被訪問的路由器都會(huì)對時(shí)間戳進(jìn)行降級處理。當(dāng)ttl值為0時(shí)，數(shù)據(jù)報(bào)被丟棄。然而對于這個(gè)方案，所有的機(jī)器都必須有同步時(shí)鐘，并且必須知道從一臺(tái)機(jī)器到另一臺(tái)機(jī)器需要多長時(shí)間?，F(xiàn)在，這個(gè)字段主要用于查詢數(shù)據(jù)報(bào)訪問的(路由器)最大跳數(shù)。當(dāng)源主機(jī)發(fā)送數(shù)據(jù)報(bào)時(shí)，它在這個(gè)字段中存儲(chǔ)一個(gè)數(shù)字。每個(gè)處理數(shù)據(jù)報(bào)的路由器將這個(gè)數(shù)字減1。如果這個(gè)值在被遞減之后為零，路由器就會(huì)丟棄數(shù)據(jù)報(bào)。這個(gè)字段是必需的，因?yàn)橐蛱鼐W(wǎng)上的路由表可能會(huì)損壞。一個(gè)數(shù)據(jù)報(bào)可能在兩個(gè)或多個(gè)路由器之間傳輸很長時(shí)間，而始終沒有到達(dá)目標(biāo)主機(jī)。所以這個(gè)字段限制了一個(gè)數(shù)據(jù)報(bào)的生命周期。

With circuit switching,a（ 1 ）path is established between two stations for communication.Switching and transmission resources within the network are（ 2 ）for the exclusive use of the circuit for the duration of the connection.The connection is（ 3 ）：Once it is established，it appears to attached devices as if there were a direct connection.Packet switching was designed to provide a more efficient facility than circuit switching for（ 4 ）data traffic.Each packet contains some portion of the user data plus control information needed for proper functioning of the network.A key distinguishing element of packet-switching networks is whether the internal operation is datagram or virtual circuit.With internal virtual circuits,a route is defined between two endpoints and all packets for that virtual circuit follow the（ 5 ）route,With internal datagrams,each packet is treated independently,and packets intended for the same destination may follow different routes.

（1） A.unique     B.dedicated     C.nondedicated     D.independent

（2） A.discarded     B.abandoned     C.reserved     D.broken

（3） A.indistinct     B.direct     C.indirect     D.transparent

（4） A.casual     B.bursty     C.limited     D.abundant

（5） A.same     B.different     C.single     D.multiple

參考答案：B、C、D、B、A

參考譯文：使用電路交換技術(shù)，在兩個(gè)站點(diǎn)之間建立一條專用的通路。在該鏈接存在期間，網(wǎng)絡(luò)內(nèi)的交換和傳輸資源完全為該電路的使用而保留。這個(gè)連接是透明的：一旦建立起連接，對與之相連的設(shè)備來說，好像存在一條直接連接一樣。分組交換可在突發(fā)通信中提供比電路交換更高效的解決方案。每個(gè)分組都由一部分用戶數(shù)據(jù)及控制信息組成，這些控制信息是網(wǎng)絡(luò)正常工作必需的。內(nèi)部操作采用的是數(shù)據(jù)報(bào)還是虛電路是區(qū)分分組交換網(wǎng)的一個(gè)關(guān)鍵要素。使用虛電路方式時(shí)，在兩個(gè)端點(diǎn)之間定義一條路由，該虛電路的所有分組都沿著同一路由前進(jìn)。使用數(shù)據(jù)報(bào)方式時(shí)，各分組被獨(dú)立處理，終點(diǎn)相同的分組可能會(huì)沿不同的路由前進(jìn)。

All three types of cryptography schemes have unique function mapping to specific.For example, the symmetric key（ 1 ）approach is typically used for the encryption of data providing（ 2 ）, whereas asymmetric key cryptography is mainly used in key（ 3 ）and nonrepudiation, thereby providing confidentiality and authentication. The hash（ 4 ）（noncryptic）， on the other hand，does not provide confidentiality but provides message integrity，and cryptographic hash algorithms provide message（ 5 ）and identity of peers during transport over insecure channels.

（1） A.Cryptography     B.decode     C.privacy     D .security

（2） A.Conduction     B.confidence      C.confidentiality     D.connection

（3） A.Authentication     B.structure     C.encryption      D.exchange

（4） A.Algorithm     B.Secure      C.structure     D.encryption

（5） A.Confidentiality     B.integrity      C.service     D.robustness

參考答案：A、C、D、A、B

參考譯文：所有三種加密方案都以其獨(dú)特的功能對應(yīng)于具體的應(yīng)用。例如，對稱密鑰加密方案通常用于保密數(shù)據(jù)的加密，而非對稱加密主要用于密鑰交換和非否認(rèn)驗(yàn)證，從而提供了保密性和認(rèn)證機(jī)制。另方面，哈希算法 (非加密的)不能提供保密性，但可以提供報(bào)文完整性檢測，而加密的哈希算法還可以提供報(bào)文完整性和對等方的標(biāo)識(shí)驗(yàn)證，這在通過不安全信道傳輸?shù)倪^程中是有用的。

Without proper safeguards, every part of a network is vulnerable to a security breach or unauthorized activity from（ 1 ），competitors, or even employees. Many of the organizations that manage their own （ 2 ） network security and use the Internet for more than just sending/receiving e-mails experience a network （ 3 ）— and more than half of these companies do not even know they were attacked. Smaller （ 4 ） are often complacent, having gained a false sense of security. They usually react to the last virus or the most recent defacing of their website. But they are trapped in a situation where they do not have the necessary time and（ 5 ）to spend on security.

（1） A.intruders     B.terminals     C.hosts     D.users

（2） A.exterior     B.internal     C.centre     D.middle

（3） A.attack     B.collapse     C.breakdown     D.virus

（4） A.users     B.campuses     C.Companies     D.networks

（5） A.safeguards     B.businesses     C.experiences     D.Resources

參考答案：A、B、A、C、D

參考譯文：如果缺乏適當(dāng)?shù)陌踩胧?網(wǎng)絡(luò)的每一部分對安全部門來說都是脆弱的,特別是遭受來自闖入者、竟?fàn)帉κ稚踔羶?nèi)部雇員的未經(jīng)授權(quán)的侵入活動(dòng)時(shí)。很多管理自己內(nèi)部網(wǎng)絡(luò)的組織,大部分都使用互聯(lián)網(wǎng),而且不僅僅是發(fā)送/接收電子郵件,這些公司都經(jīng)歷過網(wǎng)絡(luò)攻擊,大部分甚至還不知道他們被攻擊過。那些小公司還會(huì)因?yàn)樘摷俚陌踩杏X而洋洋自得。他們通常只能對最近發(fā)現(xiàn)的計(jì)算機(jī)病毒或者給他們網(wǎng)站造成的損害做出反應(yīng)。但是他們已經(jīng)陷入了沒有必要的時(shí)間和資源來進(jìn)行安全防護(hù)的困境。