Cards and chips
You'll need some computers, an Ethernet card or chip
each and some cables. Basically you have the choice between 10base2 / 10baseT
(10 MBit/s transfer rate max) and 100baseT (100 MBit/s max). Go for the latter
if possible. A bit more costly but when it comes to networks, faster is always
better. Macs come with built-in 100baseT (or even faster) Ethernet anyway.
I've made good experiences with Intel's Ethernet Pro 100 cards. Do some research
on network card
Linux compatibility when buying new cards.
Other possibilities are networking via USB or IEEE1394('FireWire'),
but you should rather stick with the true and tested when it comes to connecting
your machines.
Cables and Connectors
Buy 'twisted pair' (UTP) cables with RJ45 connectors
(look like phone connectors). Make sure those cables are 'Cat 5' (should
be printed on the box). There's nothing more embarrassing trying to troubleshoot
network problems for hours and then to discover they were due to shoddy cables.
An older variant are 'thinnet' cables with BNC connectors (look like silver
'T's). They are only capable of 10 MBit/s connections, but they don't require
a router when connecting more than two machines. On the other hand the entire
network will go down if just one connector is pulled off (not that easy,
though). Laptops with built-in Ethernet almost never have BNC connectors,
same goes for Macs.
Hubs and Switches
If you've only got two computers you want to connect
with each other with twisted pair cables, you can do so with a special 'Twisted
Pair Crossover' cable. For more than two computers you will require a hub
or even better a switch (a switch does some load balancing). These days you
can get switches the size of two matchboxes for about 30 bucks.
If you want to have it really easy, buy a hardware router
or use an obsolete computer to work as a router. A router can act as a switch,
a DHCP (or even DNS) server, a firewall, an uplink to the Internet etc.
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In order to establish communication between your machines,
you need a communication protocol which each machine understands. This communication
protocol constitutes the foundation for all high level services such as FTP,
HTTP or SMTP.
There have been multiple attempts by vendors to create
proprietary communication protocols like NetBEUI by IBM / Microsoft or IPX/SPX
by Netware. But in the end, all these attempts failed to replace the open
TCP/IP (Transmission Control Protocol / Internet Protocol), first incorporated
in 4.2BSD Unix in the early eighties. It's the same protocol which keeps
the Internet working and there's barely an operating system these days which
doesn't come with a TCP/IP stack. Perfect choice ;-).
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A communication protocol alone however will get you
nowhere. In order to establish communication, the sender has to know where
the receiver is and the receiver the location of the sender, or it will be
a very one-sided communication.
Computers love numbers, so it should be no surprise
that numbers are used to specify the communicating network interfaces (Ethernet
cards, modems etc). In short, these numbers are referred to as IP numbers
or IPs. Surely you've encountered them before: 192.168.1.145, 62.53.168.6
etc. If a computer has got several network interfaces, it can have several
IP numbers.
In Red Hat based Linux distributions like Mandrake Linux,
these IPs are assigned via interface scripts in '/etc/sysconfig/network-scripts/',
e.g. '/etc/sysconfig/network-scripts/ifcfg-eth0 ' for the first Ethernet
interface. You usually do not need to edit these files directly, you do that
via utilities like 'DrakNet'. But it's good to know where they are, just
in case ...
Every system with a working TCP/IP stack has
the internal IP 127.0.0.1, even if it's not connected to the outside world.
Do a
ping 127.0.0.1
and you will get a response if the TCP/IP system is
correctly configured. 127.0.0.1 is used for internal system purposes only
(jargon: 'local loopback', configured via '/etc/sysconfig/network-scripts/ifcfg-lo'),
you can not use it when communicating with other machines.
The IP number scheme is really messy and if you're not
a math freak, you really don't want to know about the gory details if not absolutely
necessary. The good news: you can reduce this scheme on small local networks
to some basic rules:
-
There are ranges of IPs reserved for local area
networks:
- 10.0.0.0 - 10.255.255.255
- 172.16.0.0 - 172.31.255.255
- 192.168.0.0 - 192.168.255.255
Always use numbers in these ranges when
configuring machines to communicate over a local network.
-
Keep the first three fields the same, only change
the last. If you've got three machines for example, assign 10.0.0.1, 10.0.0.2
and 10.0.0.3,not 10.0.0.1, 10.0.1.1 and 10.1.1.1. This keeps
machines in the same 'compartment' and you can use the same subnetmask (don't
ask) of 255.255.255.0 for every machine.
-
Do not assign IPs ending on 0 or
255. Choose numbers between 1 and 254. IPs ending on 0 and 255 are reserved
for special purposes.
-
Do not assign the same number twice.
IP numbers can be assigned either statically via a system
configuration file or dynamically via a DHCP (Dynamic Host Configuration
Protocol) server.
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Computers like numbers, (most) humans do not. You'd
rather type 'http://slashdot.org' than 'http://64.28.67.150' although both
lead to the same network interface. In order to allow you to enter a name,
some system is needed which converts ('maps') these names to their IPs and
vice versa.
Back in the old days, when the Internet was nothing
but a bunch of university and laboratory main frames, this was done by maintaining
a central 'HOSTS.TXT' file which was then transferred via FTP to all hosts.
This file contained single line IP / name pairs. With the explosive growth
of the number of machines connected to the Internet, this system became impractical
and was substituted by the BIND DNS (Berkeley Internet Name Domain Domain
Name System).
The 'HOSTS.TXT' system is still in use, though. Type
ping localhost
and you will get the response
PING localhost.localdomain (127.0.0.1) from 127.0.0.1
: 56(84) bytes of data.
The file responsible for this mapping is '/etc/hosts'.
In its default configuration, it will look like this:
127.0.0.1 localhost.localdomain localhost
First there's the IP number, the second field is the
'full name' of the machine and the third contains one or more aliases (short
names) for this machine.
Now you might ask: 'If the system uses both, the hosts
file for local lookups and DNS for name resolution on the Internet, what
decides which is queried first?' '/etc/hosts.conf' does this:
order hosts,bind
multi on
Every name query results in a lookup in '/etc/hosts'
first and if unsuccessful, the query is forwarded to one of the DNS servers
listed in '/etc/resolv.conf'.
You can use the 'hosts' file for setting up name resolution
in your network and that's what you will do in the practical part of this
article. While this method is easy to implement, it has a drawback: it only
works if IPs are assigned statically, that is if each network interface on
the network always gets the same address. If you've got clients which have
to use DHCP for some reasons (e.g. laptops which are often connected to other
networks), you have three possibilities:
-
Do not use name resolution for these clients. Name
resolution is optional, you can always address connected machines by their
IPs. Make sure the IP range available to DHCP clients does not overlap with
the statically assigned IPs.
-
Configure and use DNS. Cool project, but beyond
the scope of this article.
-
Configure your router to always assign the same
IP to the same network interface. Each network card or chip has a unique
identifier, its MAC address. Many routers allow you to configure to always
assign a certain IP to a certain MAC address. Check your router's documentation.
When it comes to naming the machines on your network,
you can do pretty much want you want. These names are not visible from the
outside, so they don't need to be Fully Qualified Domain Names (FQDN) consisting
of the local hostname and its domain name, including a top-level domain.
In fact, it's better if you explicitly avoid choosing existing FQDNs
on the Internet as names for your hosts since this might cause unnecessary
traffic for the machine whose FQDN you are using.
If you are at a loss what names to use, have a look
at FYI/FYI5 which contains
very good advice on this topic.
Keep in mind that naming machines is convenience, not
a necessity.
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If you follow my advice and keep all machines in the
same subnet, there isn't much to say here, except for the topic of routers.
Routers act as network gateways for all computers on
a network. As you will see in the practical part of this article, these computers
have to be configured to use the IP of the router as the destination address
for all outgoing traffic.
In Red Hat based Linux distributions, gateways are set via the GATEWAY option
in '/etc/sysconfig/network'.
It's important for your sanity to keep this in mind:
all outward bound network traffic will go through
a gateway if a gateway is configured. Donot configure a machine
to use a gateway if
- There is no router on your network;
- the machine itself acts as the router for the network;
- you want to go online directly via an interface on
this machine.
The last point may need some elaboration: A wrong gateway
configuration is possibly responsible for about 90% of all 'I can connect
to the Net but get nowhere' help messages out there. A typical route table
of a wrongly configured network looks like this (output condensed):
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Iface
217.5.98.53 0.0.0.0 255.255.255.255 UH ppp0
192.168.1.0 0.0.0.0 255.255.255.0 U eth0
127.0.0.0 0.0.0.0 255.0.0.0 U lo
0.0.0.0 192.168.1.2 0.0.0.0 UG eth0
The interface with the IP 192.168.1.2 is configured
as the gateway ('G' flag) for this machine. All network traffic (Destination
0.0.0.0) except for traffic going out to interfaces with IPs in the (local)
192.168.1. range, traffic going out to 217.5.98.53 and the local loopback
is sent to this gateway machine.
If you now type a URL in the address bar of your web browser, the request
is sent not to the 'ppp0' interface which is connected to the Internet, but
to the machine with the IP 192.168.1.2. If that machine isn't connected to
the Internet itself and configured to allow connection sharing, the request
will inevitably fail.
A correct routing table for this system would look like this
Kernel IP routing table
Destination Gateway Genmask Flags Iface
217.5.98.53 0.0.0.0 255.255.255.255 UH ppp0
192.168.1.0 0.0.0.0 255.255.255.0 U eth0
127.0.0.0 0.0.0.0 255.0.0.0 U lo
0.0.0.0 217.5.98.53 0.0.0.0 UG ppp0
Morale: If you plan to connect the machines on your
LAN to the Internet, do so via the machine which already acts as the local
router / gateway. If your network doesn't have a local gateway / router,
don't configure your machines to use one.
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 Practice
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