Because the kernel you are running now might not yet have support for the network types or cards that you wish to use, you will probably need the kernel source in order to recompile the kernel with the appropriate options.

However, as long as you stay within the mainstream of hardware, there should be no need to recompile your kernel unless there is a very specific feature that you need.

You can always obtain the latest kernel source from the sunsite.unc.edu web site. This is not the official site, but they have LOTS of bandwidth and capacity. The official site is kernel.org but please use the above if you can. Please remember that ftp.kernel.org is seriously overloaded. Use a mirror instead.

Normally, the kernel source will be untarred into the /usr/src/linux directory. For information on how to apply patches and build the kernel, you should read the Kernel-HOWTO. For information on how to configure kernel modules, you should read the Modules mini-HOWTO. Also, the README file found in the kernel sources and the Documentation directory are very informative for the brave reader.

Unless specifically stated otherwise, I recommend you stick with the standard kernel release (the one with the even number as the second digit in the version number). Development release kernels (the ones with the odd second digit) may have structural or other changes that may cause problems working with the other software on your system. If you are uncertain that you could resolve those sorts of problems, in addition to the potential for there being other software errors, then don't use them.

Internet Protocol (IP) addresses are composed of four bytes^[3]. The convention is to write addresses in what is called “dotted decimal notation”. In this form, each byte is converted to a decimal number, (0-255), dropping any leading zeroes, unless the number is zero and written with each byte separated by a “.” character. By convention, each host or router interface has an IP address. It is legal for the same IP address to be used on each interface of a single machine in some circumstances, but usually, each interface will have its own address.

Internet Protocol networks are contiguous sequences of IP addresses. All addresses within a network have a number of digits within the address in common. The portion of the address that is common amongst all addresses within the network is called the “network portion” of the address. The remaining digits are called the “host portion”. The number of bits that are shared by all addresses within a network is called the netmask and it is the latter's role to determine which addresses belong to the network it is applied to and which don't. For example, consider the following:

Any address that is “bitwise anded” with its netmask will reveal the address of the network it belongs to. The network address is therefore always the lowest numbered address within the range of addresses on the network and always has the host portion of the address coded all zeroes.

The broadcast address is a special one which every host on the network listens to, in addition to its own unique address. This address is the one that datagrams are sent to if every host on the network is meant to receive it. Certain types of data like routing information and warning messages are transmitted to the broadcast address so that every host on the network can receive it simultaneously. There are two commonly used standards for what the broadcast address should be. The most widely accepted one is to use the highest possible address on the network as the broadcast address. In the example above, this would be 192.168.110.255. For some reason, other sites have adopted the convention of using the network address as the broadcast address. In practice, it doesn't matter very much which you use, but you must make sure that every host on the network is configured with the same broadcast address.

For administrative reasons, some time early in the development of the IP protocol, some arbitrary groups of addresses were formed into networks and these networks were grouped into what are called classes. These classes provide a number of standard size networks that could be allocated. The ranges allocated are:

What addresses you should use depends on exactly what it is that you are doing. You may have to use a combination of the following activities to get all the addresses you need:

The routing configuration described above is best suited for simple network arrangements where there is only one possible path to a determined destination. When you have a more complex network arrangement, things get a little more complicated. Fortunately for most of you, this won't be an issue.

The big problem with “manual routing” or “static routing” as described, is that if a machine or link fails in your network, then the only way you can direct your datagrams another way, if another way exists, is by manually intervening and executing the appropriate commands. Naturally this is clumsy, slow, impractical and hazard prone. Various techniques have been developed to automatically adjust routing tables in the event of network failures where there are alternate routes. All of these techniques are loosely grouped by the term “dynamic routing protocols”.

You may have heard of some of the more common dynamic routing protocols. The most common are probably RIP (Routing Information Protocol) and OSPF (Open Shortest Path First Protocol). The Routing Information Protocol is very common on small networks such as small-to-medium size corporate networks or building networks. OSPF is more modern and more capable of handling large network configurations and better suited to environments where there is a large number of possible paths through the network. Common implementations of these protocols are: routed - RIP and gated - RIP, OSPF and others. The routed program is normally supplied with your Linux distribution or is included in the “NetKit” package detailed above.

An example of where and how you might use a dynamic routing protocol might look something like Figure 11.1.

Figure 11.1. A Dynamic Routing Example

We have three routers: A, B and C. Each one supports one Ethernet segment with a Class C IP network (netmask 255.255.255.0). Each router also has a PPP link to each of the other routers. The network forms a triangle.

It should be clear that the routing table at router A could look like:

	root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
	root# route add -net 192.168.2.0 netmask 255.255.255.0 ppp0
	root# route add -net 192.168.3.0 netmask 255.255.255.0 ppp1
	

This would work just fine until the link between router A and B should fail. If that link fails, then with the routing entry shown above, hosts on the Ethernet segment of A could not reach hosts on the Ethernet segment on B because their datagram would be directed to router A's ppp0 link, which is broken. They could still continue to talk to hosts on the Ethernet segment of C and hosts on the C's Ethernet segment could still talk to hosts on B's Ethernet segment, because the link between B and C is still intact.

But wait, if A can talk to C and C can still talk to B, why shouldn't A route its datagrams for B via C and let C send them to B? This is exactly the sort of problem that dynamic routing protocols like RIP were designed to solve. If each of the routers A, B and C were running a routing daemon, then their routing tables would be automatically adjusted to reflect the new state of the network should any one of the links in the network fail. To configure such a network is simple. For each router, you only need to do two things. In this case, for router A:

root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
root# /usr/sbin/routed
	

The routed routing daemon automatically finds all active network ports when it starts and sends and listens for messages on each of the network devices, to allow it to determine and update the routing table on the host.

This has been a very brief explanation of dynamic routing and where you would use it. If you want more information, then you should refer to the suggested references listed at the top of the document.

The important points relating to dynamic routing are: