Running Linux, 5th Edition - Matthias Kalle Dalheimer [246]
127.0.0.1 127.0.0.1 255.0.0.0 UH 3584 0 0 lo
128.17.75.98 127.0.0.1 255.255.255.0 UH 3584 0 0 lo
As you can see, papaya is connected to the 128.17.75 network through its eth0 device and to 128.17.112 through eth1. The default route is through pineapple, which is a gateway to the Wild Blue Yonder (as far as papaya is concerned).
Once papaya receives a packet destined for pear, it sees that the destination address is on the network 128.17.112 and routes that packet to the network using the second entry in the routing table.
Similarly, if eggplant wanted to send packets to machines outside the local organization, it would route packets through papaya (its gateway). papaya would, in turn, route outgoing packets through pineapple, and so forth. Packets are handed from one gateway to the next until they reach the intended destination network. This is the basic structure upon which the Internet is based: a seemingly infinite chain of networks, interconnected via gateways.
Hardware Requirements
You can use Linux TCP/IP without any networking hardware ; configuring "loopback" mode allows you to talk to yourself. This is necessary for some applications and games that use the loopback network device.
However, if you want to use Linux with an Ethernet TCP/IP network, you'll need an Ethernet adapter card. Many Ethernet adapters are supported by Linux for the ISA, EISA, and PCI buses, as well as USB and PCMCIA adapters. See the Linux Ethernet HOWTO for a complete discussion of Linux Ethernet hardware compatibility. For any reasonably new computer (that was sold in the last, say, two to three years), it is also quite likely that the computer has Ethernet built in, so you do not have install an Ethernet adapter card. You'll recognize this because there is an Ethernet connector socket (type RJ45) somewhere.
Over the last few years, support has been added for non-Ethernet high-speed networking, such as HIPPI. This topic is beyond the scope of this book, but if you are interested, you can get some information from the directory Documentation/networking in your kernel sources.
If you have an ADSL connection and use an ADSL router, this looks to Linux just like a normal Ethernet connection. As such, you need neither specific hardware (except an Ethernet card, of course) nor special drivers besides the Ethernet card driver itself. If you want to connect your Linux box directly to your ADSL modem, you still don't need to have any particular hardware or driver, but you do need to run a protocol called PPPoE (PPP over Ethernet); more about this later.
Linux also supports SLIP and PPP, which allow you to use a modem to access the Internet over a phone line. In this case, you'll need a modem compatible with your SLIP or PPP server; for example, many servers require a 56-kbps V.90 modem (most also support K56flex). In this book, we describe the configuration of PPP because it is what most Internet service providers offer.
Finally, there is PLIP, which lets you connect two computers directly via parallel ports, requiring a special cable between the two.
Configuring TCP/IP with Ethernet
In this section, we discuss how to configure an Ethernet TCP/IP connection on a Linux system. Presumably this system will be part of a local network of machines that are already running TCP/IP; in this case, your gateway, nameserver, and so forth are already configured and available.
The following information applies primarily to Ethernet connections. If you're planning to use PPP, read this section to understand the concepts, and follow the PPP-specific instructions in "Dial-Up PPP," later in this chapter.
On the other hand, you may wish to set up an entire LAN of Linux machines (or a mix of Linux machines and other systems). In this case, you'll have to take care of a number of other issues not discussed here. This includes setting up a nameserver for yourself, as well as a gateway machine if your network is to be connected to other networks. If your network is to be connected to the Internet, you'll also have