>> Two very important network services that office admins, systems administrators,deal with on a daily basis, are DHCP,Dynamic Host Configuration Protocol, and DNS, Domain Name System.
>>办公管理员、系统管理员每天要处理的两个非常重要的网络服务是DHCP、动态主机配置协议和DNS、域名系统。
In this module, we'll explore both.
在本模块中，我们将同时研究这两个问题。
Back in the early days of networking, devices were statically, manually configuredwith their 32-bit IPv4 addresses and subnet masks.
在网络的早期，设备是静态的，手动配置32位IPv4地址和子网掩码。
That means an administrator had to walk over to each and every machine, access the settings,change the settings, and maintain the settings.
这意味着管理员必须遍历每台机器、访问设置、更改设置并维护设置。
Humans are very prone to errors, one mistake and the IP address or subnet mask could meanthat the device could not have any network connectivity.
人很容易出错，一个错误和IP地址或子网掩码可能意味着设备不能有任何网络连接。
This wasn't very scalable either, there needed to be a better way.
这也不是很容易扩展，需要有更好的方法。
Reverse Address Resolution Protocol, RARP, was one early solution for diskless work stations.
反向地址解析协议RARP是无磁盘工作站的一种早期解决方案。
If this sounds like the ARP we've seen at great lengthin this course, well, it is the reverse of ARP.
如果这听起来像我们在这门课上看到的ARP，它是ARP的反方向。
A diskless work station would still have a NIC, and therefore a Mac address,and would ask a Reverse ARP server for an IP address.
一个无磁盘的工作站仍然有一个网卡，因此有一个Mac地址，并要求反向ARP服务器提供一个IP地址。
Administrators would pre-configure a table on the Reverse ARP server matching Mac addressesof devices to the IP addresses that would be dynamically handed out to them upon request.
管理员将在反向ARP服务器上预先配置一个表，将设备的Mac地址与在请求时动态分发给他们的IP地址相匹配。
ARP matches an IP address to a corresponding Mac address.
ARP将IP地址与对应的Mac地址匹配。
Reverse ARP matches a Mac addressto a corresponding IP address, hence the term "Reverse ARP."
反向ARP将Mac地址匹配到相应的IP地址，因此称为“反向ARP”。
This was good for admins because they didn't have to walk over to the machinesand manually enter the configurations.
这对于管理员来说很好，因为他们不必走到机器前手动输入配置。
However, the Mac addresses still needed to be collected and entered along with the IP addressthat would be handed out on the Reverse ARP server.
然而，Mac地址仍然需要收集，并与将在反向ARP服务器上分发的IP地址一起输入。
Another problem with Rivers ARP was that like ARP,it was a layer to protocol that existed inside of frames.
Rivers ARP的另一个问题是，就像ARP一样，它是一个存在于帧内的协议层。
Since there was no IP header, it couldn't be sent off a network.
由于没有IP报头，它无法从网络发送出去。
What that meant was that each and every network needed its own Reverse ARP server.
这意味着每个网络都需要自己的反向ARP服务器。
Costs and resources were obvious issues, that called for a new protocol.
成本和资源是显而易见的问题，需要制定新的协议。
Then along came BootP, Bootstrap particle.
然后出现了BootP，引导粒子。
The advantage BootP afforded over Reverse ARP was that the BootP messages were encapsulatedin UVP data grams at layer 4, which were encapsulated in IP packets at layer 3.
与反向ARP相比，BootP的优势在于，BootP消息封装在第4层的UVP数据图中，而UVP数据图封装在第3层的IP包中。
What good is that?
这有什么好处?
Well, now these messages could actually by routed off a network and the need for a serveron each and every network was eliminated.
现在，这些信息实际上可以通过路由到一个网络上每个网络上都不需要服务器。
A machine would boot up, send a request to the BootP server with its Mac address,and it would be assigned an IP address that the admin chose for that Mac address.
一台机器将启动，发送一个带有其Mac地址的请求到BootP服务器，并将为其分配一个管理员为该Mac地址选择的IP地址。
Relay agents made this possible.
中继代理使这成为可能。
Routers don't forward broadcasts, but partof the BootP sequence involved broadcasting a request to a BootP server,because obviously the client doesn't have any knowledge of anything at that point in time.
路由器不转发广播，但是BootP序列的一部分涉及到将请求广播到BootP服务器，因为很明显客户端在那个时候不知道任何事情。
Specifically, it doesn't know who to ask, so it sends out a broadcast saying, "Hey,I need an IP address, can anyone help me?"
具体来说，它不知道该问谁，所以它会发送一个广播说，“嘿，我需要一个IP地址，有人能帮我吗?”
Relay agents were router interfaces on a network, also serving as the full gateways,that took broadcast BootP messages and turned them into unicast messages,relaying them through the normal IP routing process to the prepare BootP servers.
中继代理是网络上的路由器接口，也充当完整的网关，它接收广播的BootP消息并将其转换为单播消息，通过正常的IP路由过程将其中继到prepare BootP服务器。
The BootP servers sent responses back to the relay agents,who relayed those messages back to the clients.
BootP服务器将响应发送回中继代理，中继代理将这些消息转发回客户机。
This way, you can have two BootP servers on a single network givingout IP addresses for many other networks.
这样，您可以在一个网络上有两个BootP服务器，为许多其他网络提供IP地址。
Two and not one for full tolerance and load balancing.
两个，而不是一个完全公差和负载平衡。
This was great, administrators didn't need to statically configure clients anymore.
这很好，管理员不再需要静态地配置客户机。
And now, a server for each network wasn't required.
现在，每个网络都不需要一个服务器。
However, there was still one small problem.
然而，仍然有一个小问题。
The admins still needed to collect Mac addressesand manually associate with IP addresses on the server.
管理员仍然需要收集Mac地址并手动与服务器上的IP地址关联。
Then along came DHCP, Dynamic Host Configuration Protocol, which is what we use today.
然后出现了DHCP，动态主机配置协议，这就是我们今天使用的。
DHCP's improvement over BootP was something called scopes, which are ranges of IP addressesthat are used in a dynamic fashion.
DHCP对BootP的改进被称为作用域，作用域是动态使用的IP地址的范围。
A client machine asks the DHCP server for an IP address and if there are addresses leftin this dynamic pool, the server picks one and assigns it to the host,binding the logical IP address to the host's physical Mac address for a duration of time.
客户机向DHCP服务器请求一个IP地址，如果这个动态池中还有剩余的地址，服务器将选择一个地址并将其分配给主机，在一段时间内将逻辑IP地址绑定到主机的物理Mac地址。
This concept is known as a lease.
这个概念被称为租赁。
Not too different than how a person might lease a car.
这和一个人租车的方式没什么不同。
Unlike a car lease, the client tries to renew the lease before it expiresso that the client doesn't lose network connectivity.
与汽车租赁不同，客户端试图在租约到期之前续订租约，这样客户端就不会失去网络连接。
The admin doesn't need to collect Mac addresses anymore, it's all dynamic!
管理员不再需要收集Mac地址，它是动态的!
In other words, the range of IP addresses that can be handed out to clients.
换句话说，可以分发给客户机的IP地址的范围。
There could even be excluded as well as reserved IP addresses in a scope.
甚至可以在一个范围内排除保留的IP地址。
DHCP was made to be an extension of BootP because of BootP's capability of relay agents.
DHCP是BootP的一个扩展，因为BootP具有中继代理的能力。
When DHCP was first introduced,router manufacturers were just introducing this relay agent capabilityinto their hardware devices for BootP.
当DHCP第一次被引入时，路由器制造商只是将这个中继代理功能引入到他们的硬件设备中用于BootP。
The designers of DHCP wanted to use this capability of relay agents for DHCP as well.
DHCP的设计者也想将这种中继代理的功能用于DHCP。
Thus, DHCP was made to be an extension of BootP so that relay agents would be ableto relay either BootP or the new DHCP messages.
因此，DHCP被做成BootP的一个扩展，这样中继代理就可以中继BootP或新的DHCP消息。
Option 53 distinguishes DHCP from BootP in the layer 7 fields.
选项53在第7层字段中将DHCP与BootP区分开来。
Nowadays, static addressing is still prevalent, specifically for servers and router interfacesthat can't rely on an external server giving them dynamic IP addresses.
现在，静态寻址仍然很流行，特别是对于不能依赖外部服务器提供动态IP地址的服务器和路由器接口。
If that server were to go down, these important resources would go down with that server.
如果服务器宕机，这些重要资源也会随之宕机。
However, statically assigning addresses is not scalable for client machines in a network.
然而，静态分配地址对于网络中的客户机来说是不可伸缩的。
转载于:https://www.cnblogs.com/sec875/articles/10380946.html
