Setting Up a Network Interface Card(NIC) & Loopback Interface(lo)

Setting Up a Network Interface Card

Loopback Interface

1.     Even if the computer is not connected to outside networks, an internal network functionality is required for some applications.

2.     This address is known as the loopback and its IP address is 127.0.0.1.

3.     You should check that this network interface is working before configuring your network cards.

4.     To do this, you can use the ifconfig utility to get some information. If you type ifconfig at a console prompt, you will be shown your current network interface configuration.

5.     If your loopback is configured, the ifconfig shows a device called lo with the address 127.0.0.1. If this device and address are not shown, you can add the device by using the ifconfig command as follows: ifconfig lo 127.0.0.1

6.     You then need to use the route command to give the system a little more information about this interface. For this you type: route add -net 127.0.0.0

7.     You now have your loopback set up and the ifconfig command shows the device lo in its listing.

Configuring the network card

1.     Configuring a network card follows the same procedure as configuring the loopback interface.

2.     You use the same command, ifconfig, but this time use the name ‘eth0’ for an Ethernet device.

3.     You also need to know the IP address, the netmask, and the broadcast addresses.

4.     These numbers vary depending on the type of network being built.

5.     For an internal network that never connects to the outside world, any IP numbers can be used, however there are IP numbers typically used with these networks.

RESERVED NETWORK NUMBERS

Network Class	Netmask	Network Addresses
A	255.0.0.0	10.0.0.0–10.255.255.255
B	255.255.0.0	172.16.0.0–17.31.255.255
C	255.255.255.0	192.168.0.0–192.168.255.255

6.     If you are connecting to an existing network, you must have its IP address, netmask, and broadcast address. You also need to have the router and domain name server addresses.

7.     In this example, you configure an Ethernet interface for an internal network. You need to issue the command:
ifconfig eth0 192.168.1.1 netmask 255.255.255.0 broadcast 192.168.1.255

8.     The result of above is file get created in /etc/sysconfig/network-scripts called ifcfg-etho

9.     We can check this file by issuing following command  :
[root@main~]# cat /etc/sysconfig/network-scripts/ifcfg-etho

Note : A broadcast address is a logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. A message sent to a broadcast address is typically received by all network-attached hosts, rather than by a specific host.

Configuring an internal network

1.     Now you have a network device configured for one computer, to add additional computers to your network you need to repeat this process on the other computers you want to add.

2.     The only change is that you need to assign a different IP address. For example, the second computer on your network could have the address 192.168.1.2, the third could have 192.168.1.3, and so on.

3.     In addition to configuring the network cards on each of the computers in the network, three files on each computer need to be modified. These files are all located in the /etc directory and they are:

                                                              i.      /etc/hosts

                                                            ii.      /etc/hosts.conf

                                                          iii.      /etc/resolv.conf

4.     The /etc/hosts.conf file contains configuration information for the name resolver and should contain the following:
order hosts, bind   multi on

5.     This configuration tells the name resolver to check the /etc/hosts file before attempting to query a nameserver and to return all valid addresses for a host found in the /etc/hosts file instead of just the first.

6.     The /etc/hosts file contains the names of all the computers on the local network.

7.     For a small network, maintaining this file is not difficult, but for a large network keeping the file up to date is often impractical.

The /etc/resolv.conf file provides information about name servers
employed to resolve hostnames.
[root@main~]# cat /etc/resolv.conf
search rcn.com
nameserver 192.168.2.1