Basic Steps in Debuging a TCP/IP Connection
 
 
  1.  Is there a physical connection?
    Is the "little green light" at the back of the machine next to where the
    network cable connects lighted/and or blinking?
    If the answer is Yes, then there is probably a physical connection.
    NOTE: on the lab machines there are two physical interfaces -
    a) the Intel EtherExpressPro 100 interface on the main board
    and
    b) the 3-Com 3c905 interface on a card in a PCI slotZ
    The cable must be plugged into the expansion slot
  2. Does the system have an IP address?
    Use the command 
        ifconfig -a 
    to check for an IP address. If you are using DHCP the addresses
    assigned by the lab DHCP server are in the range 192.168.0.100-200
  3. If the answer to the above is no, then either the dhcp client on your
    machine is not running or the correct module may not be installed
    in the kernel. (You need to instal a kernel module to support your
    network interface.)
  4. To check the state of the kernel modules use the following command:
        lsmod
    This lists all of the modules presently installed in the kernel.
  5. If the 3c59x module is not listed use one of the following commands
    to install this module.
        insmod 3c59x
            or
        modprobe 3c59x
    If these commands report errors, (e.g. module not found) Then you will 
    need to look in 
    /lib/modules/<kernel_version_number>/kernel/drivers/net
    for the file 3c59x.o
    If it is not there then you will need to build the kernel modules, but this
    is not a common issue in the labs.
  6. To start the DHCP client use the following command:
    /sbin/dhcpcd
    Note: There is useful information in the /etc/rc.d/rc.inet1 file
  7. Try the ifconfig command again. If there is no valid IP address then you
    will need to do it manually.
    ifconfig eth0 192.168.0.<PC_Number>
    Will do the job - use the number from the white sticker on your PC where it
    says <PC_Number>. This ensures that each machine has a unique number
    and that they do not clash with the DHCP assigned addresses. It is not
    necessary to specify the subnet mask of 255.255.255.0 since that is assumed
    by ifconfig since 192.168.0.0 is a class C network address.
Differences between insmod and modprobe:
insmod and modprobe both install modules into a running kernel. The primary
difference and advantage of modprobe is that it also checks for dependencies and
installs them as well. If module foo needs module bar installed first then:
insmod foo
will report that module bar is not installed and it will not install foo.
modprobe foo
would simply install both required modules. This may be what you want, but
it also means that you may not be aware that foo requires bar. Not knowing
this may be a problem later.

modprobe is also intended as an "all in one" module tool. insmod is more of a
conventional Unix tool that only does a single task.

References - see:
man insmod
man modprobe
man ifconfig
man dhcpcd
/etc/rc.d/rc.inet1