Installing the Binaries
If you are using one of the prepackaged Linux distributions, it will contain the major networking applications and utilities along with a coherent set of sample files. The only case in which you might have to obtain and install new utilities is when you install a new kernel release. As they occasionally involve changes in the kernel networking layer, you will need to update the basic configuration tools. This update at least involves recompiling, but sometimes you may also be required to obtain the latest set of binaries. These binaries are available at their official home site at ftp.inka.de/pub/comp/Linux/networking/NetTools/, packaged in an archive called net-tools-XXX.tar.gz, where XXX is the version number. The release matching Linux 2.0 is net-tools-1.45.
If you want to compile and install the standard TCP/IP network applications yourself, you can obtain the sources from most Linux FTP servers. All modern Linux distributions include a fairly comprehensive range of TCP/IP network applications, such as World Wide Web browsers, telnet and ftp programs, and other network applications, such as talk. If you do find something that you do need to compile yourself, the chances are good that it will compile under Linux from source quite simply if you follow the instructions included in the source package.
Setting the Hostname
Most, if not all, network applications rely on you to set the local host's name to some reasonable value. This setting is usually made during the boot procedure by executing the hostname command. To set the hostname to name, enter:
# hostname name
It is common practice to use the unqualified hostname without specifying the domain name. For instance, hosts at the Virtual Brewery (described in Appendix A, Example Network: The Virtual Brewery) might be called vale.vbrew.com or vlager.vbrew.com. These are their official fully qualified domain names (FQDNs). Their local hostnames would be the first component of the name, such as vale. However, as the local hostname is frequently used to look up the host's IP address, you have to make sure that the resolver library is able to look up the host's IP address. This usually means that you have to enter the name in /etc/hosts.
Some people suggest using the domainname command to set the kernel's idea of a domain name to the remaining part of the FQDN. This way you could combine the output from hostname and domainname to get the FQDN again. However, this is at best only half correct. domainname is generally used to set the host's NIS domain, which may be entirely different from the DNS domain to which your host belongs. Instead, to ensure that the short form of your hostname is resolvable with all recent versions of the hostname command, either add it as an entry in your local Domain Name Server or place the fully qualified domain name in the /etc/hosts file. You may then use the -fqdn argument to the hostname command, and it will print the fully qualifed domain name.
Assigning IP Addresses
If you configure the networking software on your host for standalone operation (for instance, to be able to run the INN Netnews software), you can safely skip this section, because the only IP address you will need is for the loopback interface, which is always 127.0.0.1.
Things are a little more complicated with real networks like Ethernets. If you want to connect your host to an existing network, you have to ask its administrators to give you an IP address on this network. When setting up a network all by yourself, you have to assign IP addresses yourself.
Hosts within a local network should usually share addresses from the same logical IP network. Hence, you have to assign an IP network address. If you have several physical networks, you have to either assign them different network numbers, or use subnetting to split your IP address range into several subnetworks. Subnetting will be revisited in the next section, "Creating Subnets".
When picking an IP network number, much depends on whether you intend to get on the Internet in the near future. If so, you should obtain an official IP address now. Ask your network service provider to help you. If you want to obtain a network number, just in case you might get on the Internet someday, request a Network Address Application Form from hostmaster@internic.net, or your country's own Network Information Center, if there is one.
If your network is not connected to the Internet and won't be in the near future, you are free to choose any legal network address. Just make sure no packets from your internal network escape to the real Internet. To make sure no harm can be done even if packets did escape, you should use one of the network numbers reserved for private use. The Internet Assigned Numbers Authority (IANA) has set aside several network numbers from classes A, B, and C that you can use without registering. These addresses are valid only within your private network and are not routed between real Internet sites. The numbers are defined by RFC 1597 and are listed in Table 2.1 in Chapter 2, Issues of TCP/IP Networking. Note that the second and third blocks contain 16 and 256 networks, respectively.
Picking your addresses from one of these network numbers is not only useful for networks completely unconnected to the Internet; you can still implement a slightly more restricted access using a single host as a gateway. To your local network, the gateway is accessible by its internal IP address, while the outside world knows it by an officially registered address (assigned to you by your provider). We come back to this concept in connection with the IP masquerade facility in Chapter 11, IP Masquerade and Network Address Translation.
Throughout the remainder of the book, we will assume that the brewery's network manager uses a class B network number, say 172.16.0.0. Of course, a class C network number would definitely suffice to accommodate both the Brewery's and the Winery's networks. We'll use a class B network here for the sake of simplicity; it will make the subnetting examples in the next section of this chapter a little more intuitive.
Creating Subnets
To operate several Ethernets (or other networks, once a driver is available), you have to split your network into subnets. Note that subnetting is required only if you have more than one broadcast network - point-to-point links don't count. For instance, if you have one Ethernet, and one or more SLIP links to the outside world, you don't need to subnet your network. This is explained in more detail in Chapter 7, Serial Line IP.
To accommodate the two Ethernets, the Brewery's network manager decides to use 8 bits of the host part as additional subnet bits. This leaves another 8 bits for the host part, allowing for 254 hosts on each of the subnets. She then assigns subnet number 1 to the brewery, and gives the winery number 2. Their respective network addresses are thus 172.16.1.0 and 172.16.2.0. The subnet mask is 255.255.255.0.
vlager, which is the gateway between the two networks, is assigned a host number of 1 on both of them, which gives it the IP addresses 172.16.1.1 and 172.16.2.1, respectively.