Howto: Ubuntu Linux convert DHCP network configuration to static IP configuration

My friend wanted to know how to change or convert DHCP network configuration to static configuration. After initial installation, he wanted to change network settings. Further, his system is w/o GUI system aka X Windows. Here is quick way to accomplish the same:

Your main network configuration file is /etc/network/interfaces

Desired new sample settings:
=> Host IP address 192.168.1.100
=> Netmask: 255.255.255.0
=> Network ID: 192.168.1.0
=> Broadcast IP: 192.168.1.255
=> Gateway/Router IP: 192.168.1.254
=> DNS Server: 192.168.1.254

Open network configuration file
$ sudo vi /etc/network/interfacesOR$ sudo nano /etc/network/interfaces

Find and remove dhcp entry:
iface eth0 inet dhcp

Append new network settings:

iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.254

Save and close the file. Restart the network:
$ sudo /etc/init.d/networking restart
Task: Define new DNS servers

Open /etc/resolv.conf file
$ sudo vi /etc/resolv.conf

You need to remove old DNS server assigned by DHCP server:
search myisp.com
nameserver 192.168.1.254
nameserver 202.54.1.20
nameserver 202.54.1.30

Save and close the file.
Task: Test DNS server

$ host cyberciti.biz
Network command line cheat sheet

You can also use commands to change settings. Please note that these settings are temporary and not the permanent. Use above method to make network changes permanent or GUI tool as described below.
Task: Display network interface information

$ ifconfig
Task: Take down network interface eth0 / take a network interface down

$ sudo ifconfig eth0 downOR $ sudo ifdown eth0
Task: Bring a network interface eth0 up

$ sudo ifconfig eth0 upOR$ sudo ifup eth0
Task: Change IP address and netmask from command line

Activate network interface eth0 with a new IP (192.168.1.50) / netmask:
$ sudo ifconfig eth0 192.168.1.50 netmask 255.255.255.0 up
Task: Display the routing table

$ /sbin/route OR$ /sbin/route -n
Output:

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
localnet * 255.255.255.0 U 0 0 0 ra0
172.16.114.0 * 255.255.255.0 U 0 0 0 eth0
172.16.236.0 * 255.255.255.0 U 0 0 0 eth1
default 192.168.1.254 0.0.0.0 UG 0 0 0 ra0

Task: Add a new gateway

$ sudo route add default gw 172.16.236.0
Task: Display current active Internet connections (servers and established connection)

$ netstat -nat
Task: Display open ports

$ sudo netstat -tulpOR$ sudo netstat -tulpn
Task: Display network interfaces stats (RX/TX etc)

$ netstat -i
Task: Display output for active/established connections only

$ netstat -e
$ netstat -te
$ netstat -tue

Where,

* -t : TCP connections
* -u : UDP connections
* -e : Established

Task: Test network connectivity

Send ICMP ECHO_REQUEST to network hosts, routers, servers etc with ping command. This verifies connectivity exists between local host and remote network system:
$ ping router
$ ping 192.168.1.254
$ ping cyberciti.biz

See simple Linux system monitoring with ping command and scripts for more information.
Task: Use GUI (Graphical Configuration) network Tool

If you are new, use GUI configuration tool, type the following command at terminal:
$ network-admin &

Above command is Ubuntu's GUI for configuring network connections tool.
Final tip - Learn how find out more information about commands

A man page is your best friend when you wanted to learn more about particular command or syntax. For example, read detailed information about ifconfig and netstat command:
$ man ifconfig
$ man netstat

Just get a short help with all command options by appending --help option to each command:
$ netstat --help

Find out what command is used for particular task by searching the short descriptions and manual page names for the keyword:
$ man -k 'delete directory'
$ apropos -s 1 remove

Display short descriptions of a command:
$ whatis rm
$ whatis netstat

Linux offers an excellent collection of utilities, which can be use to finding the files and executables, remember you cannot memorize all the commands and files ;)