IP Addressing

Humans work with names. Computers work with numbers. To identify a specific machine, a unique number called an ´IP address´ is assigned to it. When the IP address is assigned by the network administrator manually, this is called a ´fixed´ or ´static´ IP address. When the network software assigns the IP address on bootstrap, it's called a ´dynamic´ IP.

Internet Protocol (IP) provides for the individual identification of resources available on a network (Printers, Firewalls, Routers, Web Servers, Mail servers etc.). IP uses a unique address for each machine or network resource. This number is a 32-bit binary address, composed of four, 8-bit numbers. IP addresses are represented as four decimal numbers between 0 and 255 separated by dots; (eg. 199.221.66.20). This is referred to as dotted-decimal notation. Anything attatched to an IP network can be assigned an IP address. Note that this means that it is possible for a single machine to have multiple IP addresses if it is running multiple networkworking applications, such as DNS, Web or Mail server software. Addresses are always unique. Because IP addresses are software configured, it is easy to move devices from one network to another simply by changing the IP address or the network mask. This process is called ´renumbering´.

When looking at an IP address, the left-most portion of the address identifies which network the mahcine (host) belongs to. The right-most portion is used as the address of the host itself. A large number of addresses in use look something like this:

EXAMPLE: CLASS 'C' ADDRESS

 Bit                         |1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|3|
Position |0|1|2|3|4|5|6|7|8|9|0|1|2|3|4|5|6|7|8|9|0|1|2|3|4|5|6|7|8|9|0|1|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                   NETWORK                     |      HOST     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Decimal  |      199      .      232      .      66       .       20      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Binary   |1 1 0 0 0 1 1 1.1 1 1 0 0 1 0 0.0 0 1 0 0 0 1 0.0 0 0 0 1 0 1 0|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In the example above, the network address is 199.232.66 and the host address is 20, the complete IP address is 199.221.66.20. IP datagrams contain source and destination addresses. Only the addresses are recorded in the packet. There is no information stored in the packet to tell us which part of the address is network and which is host. If this is true, then how would we figure out which part of the address is the network portion, and which is the host portion?

First, you must understand that all hosts on the same network will have the same network address (the network portion will be the same for all machines). Only the host portion will be different and unique for each host on the network.

Different networks have different network addresses. Network A would have a different address from Network B. From the perspective of determining the correct network, the individual host address is irrelevant. We will need it later to find the host itself ON the network, but we don't need to look at it yet, since we need to find the correct network first..

To find a particular machine, you first find the network that machine is on, then ask that network to find the host machine. There are two solutions to handling this network vs. host address problem, and they are similar but separate addressing types: classful , and classless.

Classful was the first addressing scheme developed. It helped manage the IP space and make organization of networks and hosts possible, but it could not support the growing complexity of the Internet, and wasted a lot of address space, so an new scheme was developed called Classless.