计算机系中英文翻译---传输控制协议和网际协议的研究与应用(编辑修改稿)

计算机系中英文翻译---传输控制协议和网际协议的研究与应用(编辑修改稿)内容摘要：

orld via TCP/IP. The routing process begins with an IP address that is unique to the sending end station .End stations may be assigned permanent IP addresses or they may borrow them as needed from a Dynamic Host Configuration Protocol(DHCP)server or other service. Each packet carries a source address, which under current(IPv4)specifications is 32 bits long .In its header, each packet also carries the IP address of the final destination. If the sending end station determines that the destination address is not local, the packet goes to a first－ hop router, typically one that is close and has been press signed to the sender. The router inspects the packet/39。 s IP address and performs a route table lookup to see if the destination end station resides on the local(physically connected)work, typically called an IP sub .An IP sub usually is assigned to each of the router/39。 s work interfaces. If the destination IP address is local, the router searches an internal store of IP addresses and local－ device Media Access Control(MAC)addresses. This store is known as the Address Resolution Protocol(ARP)cache. ARP is the universal tool for matching IP addresses to MAC addresses. If the destination39。 s MAC address appears, the router installs that MAC address in the packet header(removing its own MAC address because that/39。 s no longer needed)and sends the packet to the destination end XV station. In the event that the destination MAC address does not appear in the ARP cache ―it might have timed out, for instance― the router must broadcast an ARP request to the sub referenced by the packet/39。 s destination IP address. The end station with that IP address responds, sending back its MAC address .The router updates its cache, installs the new MAC address into the packet header and launches the packet. If the route table lookup shows that the packet is destined for a nonlocal sub, the router forwards the packet to the next－ hop router using the next－ hop router39。 s MAC address. Routing tables are continuously built and rebuilt by intelligent discovery protocols, such as Routing Information Protocol or Open Shortest Path First(OSPF). Each router/39。 s routing table shows the best route to the destination address。 for addresses that may be several hops away, it shows the best next－ hop router. For a TCP/IP wide area work (WAN) to work efficiently as a collection of works, the routers that pass packets of data between works do not know the exact location of a host for which a packet of information is destined. Routers only know what work the host is a member of and use information stored in their route table to determine how to get the packet to the destination host39。 s work. After the packet is delivered to the destination39。 s work, the packet is delivered to the appropriate host. For this process to work, an IP address has two parts. The first part of an IP address is used as a work address, the last part as a host address. If you take the example and divide it into these two parts you get the following: (,.132Host)or(,st address). XVI Three) Sub mask Which is required for TCP/IP to work, is the sub mask. The sub mask is used by the TCP/IP protocol to determine whether a host is on the local sub or on a remote work. In TCP/IP, the parts of the IP address that are used as the work and host addresses are not fixed, so the work and host addresses above cannot be determined unless you have more information. This information is supplied in another 32bit number called a sub mask. In this example, the sub mask is . It is not obvious what this number means unless you know that 255 in binary notation equals 11111111。 so, the sub mask is: . Lining up the IP address and the sub mask together, the work and host portions of the address can be separated: address() mask() The first 24 bits (the number of ones in the sub mask) are identified as the work address, with the last 8 bits (the number of remaining zeros in the sub mask) identified as the host address. This gives you the following: address() address() So now you know, for this example using a sub mask, that the work ID is , and the host address is . When a packet arrives on the sub (from the local sub or a remote work), and it has a destination address of , your puter will receive it from the work and process it. Almost all decimal sub masks convert to binary numbers that are all ones on the left and all zeros on the right. Some other mon sub masks are: XVII Decimal Binary Inter RFC 1878 describes the valid subs and sub masks that can be used on TCP/IP works. XVIII Four) Network classes Inter addresses are allocated by the InterNIC the organization that administers the Inter. These IP addresses are divided into classes. The。