Hardware Requirements

You can actually configure Linux to use TCP/IP without any network card or connection to a network at all, using a technique called loopback. Loopback is a method of instruction that enables parts of TCP/IP to talk to another part without leaving the machine. Essentially, you are creating a loop between a software exit and a software entry. Loopbacks are frequently used to test TCP/IP configurations, and some software requires a loopback to be in place for proper operation. A loopback driver always has the IP address 127.0.0.1.

If you want to connect your Linux machine to a network, you need a network card. Linux uses Ethernet, a network system that was designed to provide TCP/IP support. A term you’ll see often is packet, which is the bundle of data and routing instructions that is assembled by TCP/IP and Ethernet to be sent over the network cables. All messages are broken into packets, and then reassembled properly at the destination.

Most Ethernet cards available today are compatible with Linux, but you should check the documentation or vendor’s support operating system lists to make you don’t buy a network card that won’t work properly. In general, all the more popular networking cards (including Plug-and-Play models designed for Windows 95) work with Linux, although some need to be manually set to the proper IRQ and memory I/O addresses.

If you plan to do your networking over the telephone (using a serial port and a modem), you don’t need a network card, but you do need a fast modem compatible with the service you are planning to use. For example, to use SLIP (Serial Line Interface Protocol), you generally need a modem supporting at least V.32bis speeds (14.4kbps).

Configuring Linux Files

Let’s assume you have a typical PC and an Ethernet card, and you want to set up your machine to run TCP/IP over the network. In most cases, the procedure described next works. However, because there are many versions of Linux, many potential conflicts with other cards and software, and unique software requirements for some systems, this should be considered only a guide.

If your TCP/IP network doesn’t work properly after following these instructions, it’s time to carefully scan all the configuration files and error messages for a clue to the problem. Also, don’t forget about the Linux USENET newsgroups, Linux User Groups, and other sources of information about Linux from which you can get help.

To configure the TCP/IP files, you must have installed the networking software on your system. If you haven’t installed the networking portions of the distribution, you must do so before proceeding. Your kernel must also have been configured and recompiled with network support added. This is usually done during the installation process, although some users will have to force the kernel recompilation manually if they installed their software in a nonstandard manner.

First we’ll deal with the use of a network card and then look at how to change the basic process to handle SLIP over a serial port and modem.

What You Need Before You Start

Before you start modifying system files, you should take a few minutes to determine a few basic pieces of information you’ll need. It is advisable to write these down somewhere so that they will be handy when you need them, and also so that you won’t enter two different values in two files, thereby causing major problems for the system.

IP Address

First you need an IP address, a unique number for your machine. Every machine on the network has to be identified uniquely to allow proper routing. TCP/IP-based networks use 32-bit addresses to uniquely identify networks and all the devices that reside within that network. These addresses are called Internet addresses or IP addresses.

The 32 bits of the IP address are broken into four 8-bit parts. Each 8-bit part can then have valid numbers ranging from 0 to 255. In IP addresses, the four 8-bit numbers are separated by a period, a notation called dotted quad. Examples of dotted quad IP addresses are 255.25.25.16 and 147.14.123.8.

For convenience, IP addresses are divided into two parts: the network number and the device number within that network. This separation into two components allows devices on different networks to have the same host number. However, since the network number is different, the devices are still uniquely identified.

For connection to the Internet, IP addresses are assigned by the Internet Network Information Center (NIC) based on the size of the network. Anyone who wants to connect to the Internet must register with the NIC to avoid duplication of network addresses. If you don’t plan to connect to the Internet, you are free to create your own numbering scheme, although future expansion and integration with Internet-using networks can cause serious problems.

For maximum flexibility, IP addresses are assigned according to network size. Networks are divided into three categories: Class A, Class B, and Class C. The three network classes break the 32-bit IP addresses into different sizes for the network and host identifiers.

A Class A address uses one byte for the network address and three bytes for the device address, allowing more than 16 million different host addresses. Class B networks use two bytes for the network and two bytes for the host. Because 16 bits allows more than 65,000 hosts, only a few large companies will be limited by this type of class. Class C addresses have three bytes for the network and one for the number of hosts. This provides for a maximum of 254 hosts (the numbers 0 and 255 are reserved) but many different network IDs. The majority of networks are Class B and Class C.

You do have a limitation as to the first value. A Class A network’s first number must be between 0 and 127, Class B addresses are between 128 and 191, and Class C addresses are between 192 and 223. This is because of the way the first byte is broken up, with a few of the bits at the front saved to identify the class of the network. Also, you can’t use the values 0 and 255 for any part because they are reserved for special purposes.