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 [email protected], 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.
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.
Note that in this example we are using a class B network to keep things simple, but a class C network would be more realistic. With the new networking code, subnetting is not limited to byte boundaries, so even a class C network may be split into several subnets. For instance, you could use two bits of the host part for the netmask, giving you 4 possible subnets with 64 hosts on each.[28]
Writing hosts and networks Files
After you have subnetted your network, you should prepare for some simple sort of hostname resolution using the /etc/hosts file. If you are not going to use DNS or NIS for address resolution, you have to put all hosts in the hosts file.
Even if you want to run DNS or NIS during normal operation, you should have some subset of all hostnames in /etc/hosts. You should have some sort of name resolution, even when no network interfaces are running, for example, during boot time. This is not only a matter of convenience, but it allows you to use symbolic hostnames in your network rc scripts. Thus, when changing IP addresses, you only have to copy an updated hosts file to all machines and reboot, rather than edit a large number of rc files separately. Usually you put all local hostnames and addresses in hosts, adding those of any gateways and NIS servers used.[29]
You should make sure your resolver only uses information from the hosts file during initial testing. Sample files that come with your DNS or NIS software may produce strange results. To make all applications use /etc/hosts exclusively when looking up the IP address of a host, you have to edit the /etc/host.conf file. Comment out any lines that begin with the keyword order by preceding them with a hash sign, and insert the line:
order hosts
The configuration of the resolver library is covered in detail in Chapter 6, Name Service and Resolver Configuration.
The hosts file contains one entry per line, consisting of an IP address, a hostname, and an optional list of aliases for the hostname. The fields are separated by spaces or tabs, and the address field must begin in the first column. Anything following a hash sign (#) is regarded as a comment and is ignored.
Hostnames can be either fully qualified or relative to the local domain. For vale, you would usually enter the fully qualified name, vale.vbrew.com, and vale by itself in the hosts file, so that it is known by both its official name and the shorter local name.
This is an example how a hosts file at the Virtual Brewery might look. Two special names are included, vlager-if1 and vlager-if2, which give the addresses for both interfaces used on vlager:
#
# Hosts file for Virtual Brewery/Virtual Winery
#
# IP FQDN aliases
#
127.0.0.1 localhost
#
172.16.1.1 vlager.vbrew.com vlager vlager-if1 1
72.16.1.2 vstout.vbrew.com vstout
172.16.1.3 vale.vbrew.com vale
#
172.16.2.1 vlager-if2
172.16.2.2 vbeaujolais.vbrew.com vbeaujolais
172.16.2.3 vbardolino.vbrew.com vbardolino
172.16.2.4 vchianti.vbrew.com vchianti
Just as with a host's IP address, you should sometimes use a symbolic name for network numbers, too. Therefore, the hosts file has a companion called /etc/networks that maps network names to network numbers, and vice versa. At the Virtual Brewery, we might install a
