Some groups of IP addresses are reserved for use only in private networks and are not routed over the Internet. These are called private IP addresses and have the following ranges:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
Home networking equipment/devices usually are configured in the factory with an IP address in the range 192.168.1.1 to 192.168.1.255.
You may be wondering how devices using private addresses could ever access the Internet if the use of private addresses on the Internet is illegal. The situation gets even more confusing if you consider the fact that hundreds of thousands of office and home networks use these same addresses. This must cause networking confusion. Don't worry, this problem is overcome by NAT.
The localhost IP Address
Whether or not your computer has a network interface card it will have a built-in IP address with which network-aware applications can communicate with one another. This IP address is defined as 127.0.0.1 and is frequently referred to as localhost. This concept is important to understand, and will be revisited in many later chapters.
TCP/IP is a universal standard suite of protocols used to provide connectivity between networked devices. It is part of the larger OSI model upon which most data communications is based.
One component of TCP/IP is the Internet Protocol (IP) which is responsible for ensuring that data is transferred between two addresses without being corrupted.
For manageability, the data is usually split into multiple pieces or packets each with its own error detection bytes in the control section or header of the packet. The remote computer then receives the packets and reassembles the data and checks for errors. It then passes the data to the program that expects to receive it.
How does the computer know what program needs the data? Each IP packet also contains a piece of information in its header called the type field. This informs the computer receiving the data about the type of layer 4 transportation mechanism being used.
The two most popular transportation mechanisms used on the Internet are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
When the type of transport protocol has been determined, the TCP/UDP header is then inspected for the "port" value, which is used to determine which network application on the computer should process the data. This is explained in more detail later.
TCP Is a Connection-Oriented Protocol
TCP opens up a virtual connection between the client and server programs running on separate computers so that multiple and/or sporadic streams of data can be sent over an indefinite period of time between them. TCP keeps track of the packets sent by giving each one a sequence number with the remote server sending back acknowledgment packets confirming correct delivery. Programs that use TCP therefore have a means of detecting connection failures and requesting the retransmission of missing packets. TCP is a good example of a connection-oriented protocol.
How TCP Establishes A Connection
Any form of communication requires some form of acknowledgement for it to become meaningful. Someone knocks on the door to a house, the person inside asks "Who is it?", to which the visitor replies, "It's me!" Then the door opens. Both persons knew who was on the other side of the door before it opened and now a conversation can now begin.
TCP acts in a similar way. The server initiating the connection sends a segment with the SYN bit set in TCP header. The target replies with a segment with the SYN and ACK bits set, to which the originating server replies with a segment with the ACK bit set. This SYN, SYN-ACK, ACK mechanism is often called the "three-way handshake".
The communication then continues with a series of segment exchanges, each with the ACK bit set. When one of the servers needs to end the communication, it sends a segment to the other with the FIN and ACK bits set, to which the other server also replies with a FIN-ACK segment also. The communication terminates with a final ACK from the server that wanted to end the session.
This is the equivalent of ending a conversation by saying "I really have to go now, I have to go for lunch", to which the reply is "I think I'm finished here too, see you tomorrow..." The conversation ends with a final "bye" from the hungry person.
Here is a modified packet trace obtained from the tethereal program. You can clearly see the three way handshake to connect and disconnect the session.
In this trace, the sequence number represents the serial number of the first byte of data in the segment. So in the first line, a random value of 9766 was assigned to the first byte and all subsequent bytes for the connection from this host will be sequentially tracked. This makes the second byte in the segment number 9767, the third number 9768 etc. The acknowledgment number or Ack, not to be confused with the ACK bit, is the byte serial number of the next segment it expects to receive from the other end, and the total number of bytes cannot exceed the Win or window value that follows it. If data isn't received correctly, the receiver will re-send the requesting segment asking for the information to be sent again. The TCP code keeps track of all this along with the source and destination ports and IP addresses to ensure that each unique connection is serviced correctly.
UDP, TCP's "Connectionless" Cousin
UDP is a connectionless protocol. Data is sent on a "best effort" basis with the machine that sends the data having no means of verifying whether the data was correctly received by the remote machine. UDP is usually used for applications in which the data sent is not mission-critical. It is also used when data needs to be broadcast to all available servers on a locally attached network where the creation of dozens of TCP connections for a short burst of data is considered resource-hungry.
TCP and UDPPorts
The data portion of the IP packet contains a TCP or UDP segment sandwiched inside. Only the TCP segment header contains sequence information, but both the UDP and the TCP segment headers track the port being used. The source/destination port and the source/destination IP addresses of the client & server computers are then combined to uniquely identify each data flow.
Certain programs are assigned specific ports that are internationally recognized. For example, port 80 is reserved for HTTP Web traffic, and port 25 is reserved for SMTP e-mail. Ports below 1024 are reserved for privileged system functions, and those above 1024 are generally reserved for non-system third-party applications.
Usually when a connection is made from a client computer requesting data to the server that contains the data:
The client selects a random previously unused "source" port greater than 1024 and queries the server on the "destination" port specific to the application. If it is an HTTP request, the client will use a source port of, say, 2049 and query the server on port 80 (HTTP)
The server recognizes the port 80 request as an HTTP request and passes on the data to be handled by the Web server software. When the Web server software replies to the client, it tells the TCP application to respond back to port 2049 of the client using a source port of port 80.
The client keeps track of all its requests to the server's IP address and will recognize that the reply on port 2049 isn't a request initiation for "NFS", but a response to the initial port 80 HTTP query.
You need to know all the steps needed to configure IP addresses on a NIC card. Web site shopping cart applications frequently need an additional IP address dedicated to them. You also might need to add a secondary NIC interface to your server to handle data backups. Last but not least, you might just want to play around with the server to test your skills.
This section shows you how to do the most common server IP activities with the least amount of headaches.
Determining Your IP Address
Most modern PCs come with an Ethernet port. When Linux is installed, this device is called eth0. You can determine the IP address of this device with the ifconfig command.
In this example, eth0 has no IP address because this box is using wireless interface wlan0 as its main NIC. Interface wlan0 has an IP address of 192.168.1.100 and a subnet mask of 255.255.255.0
You can see that this command gives good information on the interrupts, or PCI bus ID, used by each card. On very rare occasions you might find that your NIC card doesn't work because it shares both an interrupt and memory access address with some other device. You can look at the contents of the /proc/interrupts file to get a listing of all the interrupt IRQs used by your system. In the example below we can see that there are no conflicts with each IRQ from 0 to 15 having only a single entry. Devices eth0 and eth1 use interrupts 10 and 5, respectively:
[root@bigboy tmp]# cat /proc/interrupts
CPU0
0: 2707402473 XT-PIC timer
1: 67 XT-PIC i8042
2: 0 XT-PIC cascade
5: 411342 XT-PIC eth1
8: 1 XT-PIC rtc
10: 1898752 XT-PIC eth0
11: 0 XT-PIC uhci_hcd
12: 58 XT-PIC i8042
14: 5075806 XT-PIC ide0
15: 506 XT-PIC ide1
NMI: 0
ERR: 43
[root@bigboy tmp]#
If there are conflicts, you might need to refer to the manual for the offending device to try to determine ways to either use another interrupt or memory I/O location.
Changing Your IP Address
If you wanted, you could give this eth0 interface an IP address using the ifconfig command.
[root@bigboy tmp]# ifconfig eth0 10.0.0.1 netmask 255.255.255.0 up
The "up" at the end of the command activates the interface. To make this permanent each time you boot up you'll have to add this command in your /etc/rc.local file which is run at the end of every reboot.
Fedora Linux also makes life a little easier with interface configuration files located in the /etc/sysconfig/network-scripts directory. Interface eth0 has a file called ifcfg-eth0, eth1 uses ifcfg-eth1, and so on. You can place your IP address information in these files, which are then used to auto-configure your NICs when Linux boots. See Figure 3-1 for two samples of interface eth0. One assumes the interface has a fixed IP address, and the other assumes it requires an IP address assignment using DHCP.
UNIX was originally developed circa 1969 in AT&T Bell Labs. Key developers: Dennis Richie and Ken Thompson.
Development was coupled to the invention of the C programming language, which allowed UNIX to be semi-portable to different hardware. (11,000 lines of portable C and 1000 lines of machine dependent assembler in those early days)
As discussed below, UNIX includes a kernel and a number of small components and utilities built to work with the kernel.
Circa 1974 the source was made available to selected Universities, including the U of T and especially Berkeley. This led to different "flavors" of UNIX. The code remained property of AT&T and the Universities signed non-disclosure agreements.
In about 1979 various commericial vendors began to adopt UNIX under license from AT&T. The number of flavors increased (System V, BSD, HP-UX, Solaris, IRIX, etc.).
In 1984 Richard Stallman drove the beginnings of the Open Source movement with the foundation of GNU. (GNU stands for "Gnu is Not Unix."). Later this became the Free Software Foundation. They began introduce open source products to work under UNIX.
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One of their first and biggest successes was developing a C compiler, gcc, that was superior to any commercially available ones.
Virtually all of the proprietary utilities, shells, etc. that are associated with UNIX have now been re-written by GNU as Open Source.
In 1991 21 year old Linus Torvalds wanted to buy a UNIX for his own computer but couldn't afford it. So he began writing a UNIX-like operating system called Linux. He made it Open Source.
With the Linux kernel and all of the GNU utilities available as Open Source, the GNU/Linux computing environment is at least as rich and powerful as the proprietary UNIX one.
Below we will occasionally refer to "UNIX/Linux," implying that UNIX and Linux are synonyms. From the standpoint of a user this is largely true. We should more properly refer to "UNIX-GNU/Linux" to give proper credit to GNU for their important role in the Linux computing environment.
Under the terms of the GPL, any person may obtain and change the code covered under the license, but must make those changes available at no charge to the world.
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The terms of the license means, for example, that a commercial vendor of GNU/Linux, such as Red Hat, must make their distribution available at no charge.
HOME /COMPUTER/HARDWARE/MONITOR/KEYBOARD/PRINTERS/SCANNER/MOUSE/HARD DISK DRIVE/ZIP DRIVE/PEN DRIVE/MOTHERBOARD/PROCESSORS/DVD DRIVES/CD DRIVES//RAM/ROM/ MEMORY/VIRTUAL MEMORY/
