IP Addressing -

IP addresses have 4 bytes, each of which is referred to as an octet. Since each byte in the address has 8 bits, an IP address is 32 bits long. IP addresses are usually displayed in decimal format where the value of each byte is converted from binary to decimal. This makes them easier to remember. For example, an IP address of 198.131.25.4 is much easier to remember than its binary equivalent of:

11000110.10000011.00011001.00000100

Each computer on a TCP/IP network has to have a unique, numeric IP address. The address consists of 32 bits, or 4 bytes, or 4 octets of data. In binary numbers, it looks something like this: 01111110.00000101.00000100.00001111.

The IP address is like a mailing address; some of the bits represent the network segment that the computer is on. That's like the street name of a mailing address. Other bits represent the particular host on the segment; that's like the house number.

A host is any system that has an IP address; this can be devices like network-attached printers as well as individual computers.

If you want to connect to the Internet you need to obtain addresses that are valid but not in use in the Internet. Organizations that maintain their own connection to the Internet, must apply to an Internet numbers registry that's been approved by IANA/ICANN. In North America, this is the American Registry for Internet Numbers (ARIN).

When the Internet first started, all IP addresses fell into one of three pre-defined address classes. The address class tells you which bits in the IP address are used for the network address, and which are used for host addresses.

These three categories got translated into three different IP address classes: Class A, B, and C. Only the largest companies received Class A network addresses, while small companies were limited to Class C addresses. Each class uses a different part of the 32-bit IP address space as the network portion of the address, leaving the remaining bits for use as the host address. A Class A network address uses the first byte as the network address, leaving 3 bytes for hosts. So a Class A network can support a limited number of separate network segments, but each one can contain many hosts. A Class B network address uses the first and second bytes, and a Class C network address uses the first 3 bytes to define the network address.

Later, two additional classes were defined: Class D, for sending multicasts, which are transmissions to multiple hosts, and Class E, for experimental purposes.

On most IP networks, computers have not only IP addresses, but they also have descriptive names that are easier for people to remember and use. This name is called the host name. It's a friendly name assigned to a computer that people can use instead of the numeric IP address

Address Classes and Subnets 

Class A addresses always use the left-most byte for the network address.

Class B uses the two left-most bytes for the network address.

Class C uses the three left-most bytes for the network address.

Class A Networks - Range: 1-126 - Default Subnet: 255.0.0.0
# of Subnets: 126
# of hosts per subnet: 16,777,214

Class B Networks - Range: 128-191 - Default Subnet: 255.255.0.0
# of Subnets: 16,384
# of hosts per subnet: 65,534

Class C Networks - Range: 192-223 - Default Subnet: 255.255.255.0
# of Subnets: 2,097,152
# of hosts per subnet: 254

Currently, all 126 Class A addresses and 16,384 Class B addresses are in use, so new companies are restricted to one of the 2,097,152 possible Class C addresses which can handle up to 254 hosts.

Subnets provide a solution to the growing shortage of IP addresses. Instead of assigning all 254 possible addresses to a company, subnets allow only a portion of a set of Class C addresses to be assigned to one network, leaving other addresses available for other independent networks.

The subnet mask is a binary number that the computer uses in a calculation to hide the host portion of the IP address, and leave only the network portion of the IP address.

Private Addressing:

IANA has set aside three address ranges for use by private or non-Internet connected networks. This is referred to as Private Address Space and is defined in RFC 1918.

The reserved address blocks are:

10.0.0.0 to 10.255.255.255 (10/8 prefix)
172.16.0.0 to 172.31.255.255 (172.16/12 prefix)
192.168.0.0 to 192.168.255.255 (192.168/16 prefix)

If a private network does need to connect to the Internet (or other TCP/IP networks), you can use Network Address Translation (as described in RFC 1631). The translation occurs on the router connecting the private network to the Internet. The private addresses are translated into valid IP addresses and then the packets are forwarded on to the outside world (beyond the private network).

Classless Inter-Domain Routing (CIDR) also called supernetting -

an IP addressing scheme that replaces the older system based on classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP network prefix. For example: 123.456.789.123/16

For more information on Networking Basics, the OSI Model and Server Administration:
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