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Translating by qhwdw
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Linux LAN Routing for Beginners: Part 1
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======
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Once upon a time we learned about [IPv6 routing][1]. Now we're going to dig into the basics of IPv4 routing with Linux. We'll start with an overview of hardware and operating systems, and IPv4 addressing basics, and next week we'll setup and test routing.
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### LAN Router Hardware
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Linux is a real networking operating system, and always has been, with network functionality built-in from the beginning. Building a LAN router is simple compared to building a gateway router that connects your LAN to the Internet. You don't have to hassle with security or firewall rules, which are still complicated by having to deal with NAT, network address translation, an affliction of IPv4. Why do we not drop IPv4 and migrate to IPv6? The life of the network administrator would be ever so much simpler.
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But I digress. Ideally, your Linux router is a small machine with at least two network interfaces. Linux Gizmos has a great roundup of single-board computers here: [Catalog of 98 open-spec, hacker friendly SBCs][2]. You could use an old laptop or desktop PC. You could use a compact computer, like the ZaReason Zini or the System76 Meerkat, though these are a little pricey at nearly $600. But they are stout and reliable, and you're not wasting money on a Windows license.
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The Raspberry Pi 3 Model B is great for lower-demand routing. It has a single 10/100 Ethernet port, onboard 2.4GHz 802.11n wireless, and four USB ports, so you can plug in more USB network interfaces. USB 2.0 and the slower onboard network interfaces make the Pi a bit of a network bottleneck, but you can't beat the price ($35 without storage or power supply). It supports a couple dozen Linux flavors, so chances are you can have your favorite. The Debian-based Raspbian is my personal favorite.
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### Operating System
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You might as well stuff the smallest version of your favorite Linux on your chosen hardware thingy, because the specialized router operating systems such as OpenWRT, Tomato, DD-WRT, Smoothwall, Pfsense, and so on all have their own non-standard interfaces. In my admirable opinion this is an unnecessary complication that gets in the way rather than helping. Use the standard Linux tools and learn them once.
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The Debian net install image is about 300MB and supports multiple architectures, including ARM, i386, amd64, and armhf. Ubuntu's server net installation image is under 50MB, giving you even more control over what packages you install. Fedora, Mageia, and openSUSE all offer compact net install images. If you need inspiration browse [Distrowatch][3].
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### What Routers Do
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Why do we even need network routers? A router connects different networks. Without routing every network space is isolated, all sad and alone with no one to talk to but the same boring old nodes. Suppose you have a 192.168.1.0/24 and a 192.168.2.0/24 network. Your two networks cannot talk to each other without a router connecting them. These are Class C private networks with 254 usable addresses each. Use ipcalc to get nice visual information about them:
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```
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$ ipcalc 192.168.1.0/24
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Address: 192.168.1.0 11000000.10101000.00000001. 00000000
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Netmask: 255.255.255.0 = 24 11111111.11111111.11111111. 00000000
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Wildcard: 0.0.0.255 00000000.00000000.00000000. 11111111
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=>
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Network: 192.168.1.0/24 11000000.10101000.00000001. 00000000
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HostMin: 192.168.1.1 11000000.10101000.00000001. 00000001
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HostMax: 192.168.1.254 11000000.10101000.00000001. 11111110
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Broadcast: 192.168.1.255 11000000.10101000.00000001. 11111111
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Hosts/Net: 254 Class C, Private Internet
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```
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I like that ipcalc's binary output makes a visual representation of how the netmask works. The first three octets are the network address, and the fourth octet is the host address, so when you are assigning host addresses you "mask" out the network portion and use the leftover. Your two networks have different network addresses, and that is why they cannot communicate without a router in between them.
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Each octet is 256 bytes, but that does not give you 256 host addresses because the first and last values, 0 and 255, are reserved. 0 is the network identifier, and 255 is the broadcast address, so that leaves 254 host addresses. ipcalc helpfully spells all of this out.
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This does not mean that you never have a host address that ends in 0 or 255. Suppose you have a 16-bit prefix:
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```
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$ ipcalc 192.168.0.0/16
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Address: 192.168.0.0 11000000.10101000. 00000000.00000000
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Netmask: 255.255.0.0 = 16 11111111.11111111. 00000000.00000000
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Wildcard: 0.0.255.255 00000000.00000000. 11111111.11111111
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=>
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Network: 192.168.0.0/16 11000000.10101000. 00000000.00000000
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HostMin: 192.168.0.1 11000000.10101000. 00000000.00000001
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HostMax: 192.168.255.254 11000000.10101000. 11111111.11111110
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Broadcast: 192.168.255.255 11000000.10101000. 11111111.11111111
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Hosts/Net: 65534 Class C, Private Internet
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```
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ipcalc lists your first and last host addresses, 192.168.0.1 and 192.168.255.254. You may have host addresses that end in 0 and 255, for example 192.168.1.0 and 192.168.0.255, because those fall in between the HostMin and HostMax.
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The same principles apply regardless of your address blocks, whether they are private or public, and don't be shy about using ipcalc to help you understand.
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### CIDR
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CIDR (Classless Inter-Domain Routing) was created to extend IPv4 by providing variable-length subnet masking. CIDR allows finer slicing-and-dicing of your network space. Let ipcalc demonstrate:
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```
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$ ipcalc 192.168.1.0/22
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Address: 192.168.1.0 11000000.10101000.000000 01.00000000
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Netmask: 255.255.252.0 = 22 11111111.11111111.111111 00.00000000
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Wildcard: 0.0.3.255 00000000.00000000.000000 11.11111111
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=>
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Network: 192.168.0.0/22 11000000.10101000.000000 00.00000000
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HostMin: 192.168.0.1 11000000.10101000.000000 00.00000001
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HostMax: 192.168.3.254 11000000.10101000.000000 11.11111110
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Broadcast: 192.168.3.255 11000000.10101000.000000 11.11111111
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Hosts/Net: 1022 Class C, Private Internet
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```
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The netmask is not limited to whole octets, but rather crosses the boundary between the third and fourth octets, and the subnet portion ranges from 0 to 3, and not from 0 to 255. The number of available hosts is not a multiple of 8 as it is when the netmask is defined by whole octets.
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Your homework is to review CIDR and how the IPv4 address space is allocated between public, private, and reserved blocks, as this is essential to understanding routing. Setting up routes is not complicated as long as you have a good knowledge of addressing.
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Start with [Understanding IP Addressing and CIDR Charts][4], [IPv4 Private Address Space and Filtering][5], and [IANA IPv4 Address Space Registry][6]. Then come back next week to learn how to create and manage routes.
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Learn more about Linux through the free ["Introduction to Linux" ][7]course from The Linux Foundation and edX.
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--------------------------------------------------------------------------------
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via: https://www.linux.com/learn/intro-to-linux/2018/2/linux-lan-routing-beginners-part-1
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作者：[Carla Schroder][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.linux.com/users/cschroder
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[1]:https://www.linux.com/learn/intro-to-linux/2017/7/practical-networking-linux-admins-ipv6-routing
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[2]:http://linuxgizmos.com/catalog-of-98-open-spec-hacker-friendly-sbcs/#catalog
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[3]:http://distrowatch.org/
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[4]:https://www.ripe.net/about-us/press-centre/understanding-ip-addressing
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[5]:https://www.arin.net/knowledge/address_filters.html
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[6]:https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml
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[7]:https://training.linuxfoundation.org/linux-courses/system-administration-training/introduction-to-linux
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@ -0,0 +1,103 @@
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Linux 局域网路由新手指南：第 1 部分
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======
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前面我们学习了 [IPv6 路由][1]。现在我们继续深入学习 Linux 中的 IPv4 路由的基础知识。我们从硬件概述、操作系统和 IPv4 地址的基础知识开始，下周我们将继续学习它们如何配置，以及测试路由。
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### 局域网路由器硬件
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Linux 实际上是一个网络操作系统，一直都是，从一开始它就有内置的网络功能。为将你的局域网连入因特网，构建一个局域网路由器比起构建网关路由器要简单的多。你不要太过于执念安全或者防火墙规则，对于处理 NAT 它还是比较复杂的，网络地址转换是 IPv4 的一个痛点。我们为什么不放弃 IPv4 去转到 IPv6 呢？这样将使网络管理员的工作更加简单。
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有点跑题了。从理论上讲，你的 Linux 路由器是一个至少有两个网络接口的小型机器。Linux Gizmos 是一个单片机的综合体：[98 个开放规格的目录，黑客友好的 SBCs][2]。你能够使用一个很老的笔记本电脑或者台式计算机。你也可以使用一个精简版计算机，像 ZaReason Zini 或者 System76 Meerkat 一样，虽然这些有点贵，差不多要 $600。但是它们又结实又可靠，并且你不用在 Windows 许可证上浪费钱。
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如果对路由器的要求不高，使用树莓派 3 Model B 作为路由器是一个非常好的选择。它有一个 10/100 以太网端口，板载 2.4GHz 的 802.11n 无线网卡，并且它还有四个 USB 端口，因此你可以插入多个 USB 网卡。USB 2.0 和低速板载网卡可能会让树莓派变成你的网络上的瓶颈，但是，你不能对它期望太高（毕竟它只有 $35，既没有存储也没有电源）。它支持很多种风格的 Linux，因此你可以选择使用你喜欢的版本。基于 Debian 的树莓派是我的最爱。
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### 操作系统
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你可以在你选择的硬件上安装将你喜欢的 Linux 的简化版，因为定制的路由器操作系统，比如 OpenWRT、 Tomato、DD-WRT、Smoothwall、Pfsense 等等，都有它们自己的非标准界面。我的观点是，没有必要这么麻烦，它们对你并没有什么帮助。尽量使用标准的 Linux 工具，因为你只需要学习它们一次就够了。
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Debian 的网络安装镜像大约有 300MB 大小，并且支持多种架构，包括 ARM、i386、amd64、和 armhf。Ubuntu 的服务器网络安装镜像也小于 50MB，这样你就可以控制你要安装哪些包。Fedora、Mageia、和 openSUSE 都提供精简的网络安装镜像。如果你需要创意，你可以浏览 [Distrowatch][3]。
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### 路由器能做什么
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我们需要网络路由器做什么？一个路由器连接不同的网络。如果没有路由，那么每个网络都是相互隔离的，所有的悲伤和孤独都没有人与你分享，所有节点只能孤独终老。假设你有一个 192.168.1.0/24 和一个 192.168.2.0/24 网络。如果没有路由器，你的两个网络之间不能相互沟通。这些都是 C 类的私有地址，它们每个都有 254 个可用网络地址。使用 ipcalc 可以非常容易地得到它们的这些信息：
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```
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$ ipcalc 192.168.1.0/24
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Address: 192.168.1.0 11000000.10101000.00000001. 00000000
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Netmask: 255.255.255.0 = 24 11111111.11111111.11111111. 00000000
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Wildcard: 0.0.0.255 00000000.00000000.00000000. 11111111
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=>
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Network: 192.168.1.0/24 11000000.10101000.00000001. 00000000
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HostMin: 192.168.1.1 11000000.10101000.00000001. 00000001
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HostMax: 192.168.1.254 11000000.10101000.00000001. 11111110
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Broadcast: 192.168.1.255 11000000.10101000.00000001. 11111111
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Hosts/Net: 254 Class C, Private Internet
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```
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我喜欢 ipcalc 的二进制输出信息，它更加可视地表示了掩码是如何工作的。前三个八位组表示了网络地址，第四个八位组是主机地址，因此，当你分配主机地址时，你将 “掩盖” 掉网络地址部分，只使用剩余的主机部分。你的两个网络有不同的网络地址，而这就是如果两个网络之间没有路由器它们就不能互相通讯的原因。
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每个八位组一共有 256 字节，但是它们并不能提供 256 个主机地址，因为第一个和最后一个值 ，也就是 0 和 255，是被保留的。0 是网络标识，而 255 是广播地址，因此，只有 254 个主机地址。ipcalc 可以帮助你很容易地计算出这些。
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当然，这并不意味着你不能有一个结尾是 0 或者 255 的主机地址。假设你有一个 16 位的前缀：
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```
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$ ipcalc 192.168.0.0/16
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Address: 192.168.0.0 11000000.10101000. 00000000.00000000
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Netmask: 255.255.0.0 = 16 11111111.11111111. 00000000.00000000
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Wildcard: 0.0.255.255 00000000.00000000. 11111111.11111111
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=>
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Network: 192.168.0.0/16 11000000.10101000. 00000000.00000000
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HostMin: 192.168.0.1 11000000.10101000. 00000000.00000001
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HostMax: 192.168.255.254 11000000.10101000. 11111111.11111110
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Broadcast: 192.168.255.255 11000000.10101000. 11111111.11111111
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Hosts/Net: 65534 Class C, Private Internet
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```
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ipcalc 列出了你的第一个和最后一个主机地址，它们是 192.168.0.1 和 192.168.255.254。你是可以有以 0 或者 255 结尾的主机地址的，例如，192.168.1.0 和 192.168.0.255，因为它们都在最小主机地址和最大主机地址之间。
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不论你的地址块是私有的还是公共的，这个原则同样都是适用的。不要羞于使用 ipcalc 来帮你计算地址。
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### CIDR
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CIDR（无类域间路由）就是通过可变长度的子网掩码来扩展 IPv4 的。CIDR 允许对网络空间进行更精细地分割。我们使用 ipcalc 来演示一下：
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```
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$ ipcalc 192.168.1.0/22
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Address: 192.168.1.0 11000000.10101000.000000 01.00000000
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Netmask: 255.255.252.0 = 22 11111111.11111111.111111 00.00000000
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Wildcard: 0.0.3.255 00000000.00000000.000000 11.11111111
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=>
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Network: 192.168.0.0/22 11000000.10101000.000000 00.00000000
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HostMin: 192.168.0.1 11000000.10101000.000000 00.00000001
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HostMax: 192.168.3.254 11000000.10101000.000000 11.11111110
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Broadcast: 192.168.3.255 11000000.10101000.000000 11.11111111
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Hosts/Net: 1022 Class C, Private Internet
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```
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网络掩码并不局限于整个八位组，它可以跨越第三和第四个八位组，并且子网部分的范围可以是从 0 到 3，而不是非得从 0 到 255。可用主机地址的数量并不一定是 8 的倍数，因为它是由整个八位组定义的。
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给你留一个家庭作业，复习 CIDR 和 IPv4 地址空间是如何在公共、私有和保留块之间分配的，这个作业有助你更好地理解路由。一旦你掌握了地址的相关知识，配置路由器将不再是件复杂的事情了。
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从 [理解 IP 地址和 CIDR 图表][4]、[IPv4 私有地址空间和过滤][5]、以及 [IANA IPv4 地址空间注册][6] 开始。接下来的我们将学习如何创建和管理路由器。
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通过来自 Linux 基金会和 edX 的免费课程 ["Linux 入门" ][7]学习更多 Linux 知识。
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--------------------------------------------------------------------------------
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via: https://www.linux.com/learn/intro-to-linux/2018/2/linux-lan-routing-beginners-part-1
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作者：[Carla Schroder][a]
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译者：[qhwdw](https://github.com/qhwdw)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.linux.com/users/cschroder
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[1]:https://www.linux.com/learn/intro-to-linux/2017/7/practical-networking-linux-admins-ipv6-routing
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[2]:http://linuxgizmos.com/catalog-of-98-open-spec-hacker-friendly-sbcs/#catalog
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[3]:http://distrowatch.org/
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[4]:https://www.ripe.net/about-us/press-centre/understanding-ip-addressing
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[5]:https://www.arin.net/knowledge/address_filters.html
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[6]:https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtml
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[7]:https://training.linuxfoundation.org/linux-courses/system-administration-training/introduction-to-linux
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||||||
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