Every interface on an internet must have a unique Internet address (also called an IP address). These addresses are 32-bit numbers. Instead of using a flat address space such as 1, 2, 3, and so on, there is a structure to Internet addresses. [Figure 1.5] shows the five different classes of Internet addresses.
internet上的每个接口都必须有惟一的internet地址(也称为IP地址)。这些地址是32位数字。与使用平面地址空间(如1、2、3等)不同，Internet地址有一个结构。[图1.5]显示了五个不同类别的互联网地址。
Figure 1.5. The five different classes of Internet addresses.png
These 32-bit addresses are normally written as four decimal numbers, one for each byte of the address. This is called dotted-decimal notation. For example, the class B address of the author's primary system is 140.252.13.33.
The easiest way to differentiate between the different classes of addresses is to look at the first number of a dotted-decimal address. [Figure 1.6] shows the different classes, with the first number in boldface.
这些32位地址通常被写成四个十进制数字，每个字节对应一个地址。这被称为点分十进制记数法。例如，作者的主系统的B类地址是140.252.13.33。
区分不同类别地址的最简单方法是查看小数点后地址的第一个数字。图1.6显示了不同的类，第一个数字以粗体显示。
It is worth reiterating that a multihomed host will have multiple IP addresses: one per interface.
Since every interface on an internet must have a unique IP address, there must be one central authority for allocating these addresses for networks connected to the worldwide Internet. That authority is the Internet Network Information Center, called the InterNIC. The InterNIC assigns only network IDs. The assignment of host IDs is up to the system administrator.
值得重申的是，多主机将有多个IP地址:每个接口一个。
由于internet上的每个接口都必须有一个惟一的IP地址，因此必须有一个中央权威机构为连接到全球internet的网络分配这些地址。那个权威机构就是互联网网络信息中心，叫做互联网络。internet只分配网络id。主机id的分配由系统管理员决定
Registration services for the Internet (IP addresses and DNS domain names) used to be handled by the NIC, at <tt class="monofont">nic.ddn.mil.</tt> On April 1,1993, the InterNIC was created. Now the NIC handles these requests only for the Defense Data Network (DDN). All other Internet users now use the InterNIC registration services, at rs.internic.net.
Internet (IP地址和DNS域名)的注册服务以前由NIC负责，1993年4月1日，在nic.ddn.mil.， Internet诞生了。现在NIC仅为“防御数据网络”(DDN)处理这些请求。所有其他互联网用户现在使用互联网注册服务，在rs.internic.net。
There are actually three parts to the InterNIC: registration services (rs.internic.net), directory and database services (ds.internic.net), and information services (is.internic.net). See Exercise 1.8 for additional information on the InterNIC.
internet实际上有三个部分:注册服务(rs.internic.net)、目录和数据库服务(ds.internic.net)以及信息服务(is.internic.net)。有关internet的更多信息，请参见练习1.8。
There are three types of IP addresses: unicast (destined for a single host), broadcast (destined for all hosts on a given network), and multicast (destined for a set of hosts that belong to a multicast group). Chapters 12 and 13 look at broadcasting and multicasting in more detail.
IP地址有三种类型:单播(针对单个主机)、广播(针对给定网络上的所有主机)和组播(针对属于组播组的主机集)。第12章和第13章更详细地介绍了广播和多播。
In Section 3.4 we'll extend our description of IP addresses to include subnetting, after describing IP routing. Figure 3.9 shows the special case IP addresses: host IDs and network IDs of all zero bits or all one bits.
在3.4节中，在描述IP路由之后，我们将扩展对IP地址的描述，以包括子网。图3.9显示了特殊情况下的IP地址:所有0位或所有1位的主机id和网络id。
1.5 The Domain Name System
Although the network interfaces on a host, and therefore the host itself, are known by IP addresses, humans work best using the name of a host. In the TCP/IP world the Domain Name System (DNS) is a distributed database that provides the mapping between IP addresses and hostnames. Chapter 14 looks into the DNS in detail.
For now we must be aware that any application can call a standard library function to look up the IP address (or addresses) corresponding to a given hostname. Similarly a function is provided to do the reverse lookup— given an IP address, look up the corresponding hostname.
1.5域名系统
虽然主机上的网络接口(因此主机本身也是IP地址)是已知的，但是人类使用主机名工作得最好。在TCP/IP世界中，域名系统(DNS)是一个分布式数据库，它提供IP地址和主机名之间的映射。第14章详细介绍了DNS。
Most applications that take a hostname as an argument also take an IP address. When we use the Telnet client in Chapter 4, for example, one time we specify a hostname and another time we specify an IP address.
大多数使用主机名作为参数的应用程序也使用IP地址。例如，当我们在第4章中使用Telnet客户机时，一次指定主机名，另一次指定IP地址。
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