Lucky With LAN

• Working with a LAN (Local Area Network) requires installing a LAN card in your computer.
  
  
• Usually, the LAN you are connecting to is of type 'Ethernet', and requires an Ethernet controller (or Ethernet NIC - Network Interface Card).
  
  
• After installing and configuring the Ethernet controller, we need to set up a means for our machine to be identified on the network.
  
  
• On some networks, each machine is given a static address. On other networks, addresses are set dynamically, as the machine boots.
  
  
• Finally, we might want to mess a bit with routing information. This is usually not required on end-user machines, though.
  
  

• For each Ethernet controller - there is an Ethernet driver that is required to control it. This driver is part of the kernel. Usually - a separate module (configured using the '/etc/conf.modules' or '/etc/modules.conf' file, depending on your kernel versions and module utils (modutils) versions).
  
  
• A common problem with Using Ethernet drivers is detection problems during installation, that lead to the system loading the wrong driver module.
  
  
• Note: with today's Linux distributions, PCI plug & play Ethernet controllers are often detected and configured automatically by the system installation process, with no problems.
  
  
• If you have an older Ethernet card, you might need to configure its IRQ and I/O address manually, using jumpers or dip-switches on the controller itself. After that, you should tell your system the exact type of Ethernet controller you have, and if you are lucky, the kernel will detect the IRQ and I/O address on its own.
  
  
• The Ethernet HOWTO contains more details on setting up Ethernet controllers.
  
  

• Each computer connected to the Internet (or any TCP/IP network) is identified by an address, called "IP address".
  
  
• IP addresses are written as a set of numbers (also called "octets"), separated by dots. For example: 192.217.24.5
  
  
• IP addresses belong to networks, or sub-networks. IP addresses assigned to machines on the same LAN should belong to the same sub-network.
  
  
• In order to specify the sub-network an IP address belongs to, we use a net-mask. It looks something like: 255.255.255.0
  
  
• The net-mask defines which part of the IP address represents the network address, and which part represents the host address.
  
  
• In our example above, the network address would be 192.217.24.0
  
  
• Conclusions: all machines connected to the same LAN, should have the same netmask, and IP addresses that begin with the same prefix.
  
  

In order to actually configure the Ethernet controller's IP address (assuming a static IP here), one may use the 'ifconfig' command (as user root, of-course). Here is how:

[root@simey ~]# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:01:02:9D:EB:92
          inet addr:192.217.24.5  Bcast:192.217.24.255  Mask:255.255.255.0
          UP BROADCAST RUNNING  MTU:1500  Metric:1
          RX packets:2256 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2868 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          Interrupt:12 Base address:0xec00

• When we configure a network device, the system automatically sets up routing to the network this device connects to, to handle all IP addresses that belong to the sub-net this device is configured for.
  
  
• In human speak, this means that if we set up an Ethernet controller with an IP address of 192.217.24.5 and a netmask of 255.255.255.0, then our machine automatically knows that communicating with machines whose IP address begins with '192.217.24.' should be done on the LAN, directly via our Ethernet controller.
  
  
• In order to allow our system to communicate with machines that are not part of our LAN, we need to set up more routing information.
  
  
• Normally, an end-user machine only needs to know that communications to non-local machines should go through a gateway machine. This gateway machine is connected both to our LAN, and to other networks, and routes communication data between our LAN and those other networks.
  
  
• This setup is known as "default gateway". This means, we tell our system that communications with any machine not found on our LAN should be done via this gateway.
  
  

In order to view our system's routing table (the table defining how to communicate with various systems), we may use the 'netstat -r' command:

[choo@simey ~]$ netstat -r
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.217.24.5    *               255.255.255.255 UH        0 0          0 eth0
192.217.24.0    *               255.255.255.0   U         0 0          0 eth0
127.0.0.0       *               255.0.0.0       U         0 0          0 lo
default         192.217.24.254  0.0.0.0         UG        0 0          0 eth0

• As mentioned above, on some networks, IP addresses (and net-masks) are assigned dynamically to machines.
  
  
• This assignment is often done using DHCP (Dynamic Host Control Protocol). In this protocol, a machine that boots up, sends a broadcast message on its LAN, asking for its IP address. A DHCP server reads this request, and sends a reply, containing the machine's IP address, netmask and default gateway.
  
  
• On Linux, we can use the 'pump' command in order to become a DHCP client. All we need to do is launch the 'pump' command (either during the system startup process, or at the prompt of user 'root'), and it will handle the DHCP process. For more information - man pump. One can also use the 'dhcpcd' (note the 'cd', for client) daemon to get a DHCP address. for more more information, man 'dhcpcd'.