Internet Protocol address (IP address)

Each machine, which is connected to the Internet, has its own IP address. The IP address is used to identify it to the world and plays an important part in the TCP/IP protocol. Usually, an IP address is displayed as a group of numbers, separated by dots.

Abstraction only understood by software
Independent of hardware addressing
Used by TCP/IP protocols and other application programs
32-bit integer
Does not identify a specific computer
Identifies a connection between a computer and a network
Unique value for each host (global IP verse local IP address)
May have more than one IP addresses for a computer with multiple network connection, e.g., a router
Classful IP addressing
Subnet and Classless IP addressing
Assigned by global authority (IANA & ISP) and local administrator

The numbers in the IP address are called octets. The name "octets" derives from the positions the numbers can have in the binary form of the IP address, which is actually used by computers. The "215.65.1.240" type of address is used for human understanding.

The octets in the IP address play another important role – they divide the IP addresses into classes. They are split into two sections – net and host. The Net section always starts with the first octet and is used to define the network the machine belongs to. The Host section defines the actual machine in the network and always contains the last octet.

The first few bits of each IP address indicate which of the address class formats it is using. The address structures look like this:

Class A

Network (7 bits)

Local address (24 bits)

Class B

Network (14 bits)

Local address (16 bits)

Class C

110

Network (21 bits)

Local address (8 bits)

Class D

1110	Multicast address (28 bits)

The IP address is usually expressed as four decimal numbers, each representing eight bits, separated by periods.

The class A IP addresses have a first or Net octet in the range of 1 and 126. The other three octets define the hosts. The A class network allows for a total of 2,147,483,648 unique IP addresses and is mainly used for the network of a very large corporation.

The class B IP addresses use the first two octets as network identifiers, and the last two as host identifiers. The first octet in the class B IP addresses is in the 128 to 191 range. IPs from this class are most commonly used by mid-sized networks, such as college campuses

For class C, the Net identifier is composed of the first 3 octets and the first octet is in the 192 to 223 range. The last octet is used to identify the host. With a limited number of hosts, this IP class is suitable for small to mid-sized networks.

IP addresses from D class are mostly used for multicasting. They are in the 224.0.0.0 to 239.255.255.255 range. The 224.0.0.0 to 224.0.0.255 range is used only for local area network (LAN) multicasting.

IP addresses in E class have the first octet in the 240-255 range. They are reserved for experimental usage and computers, trying to use them will not be able to communicate properly online.

When looking at a standard IPv4 address, one can easily see that it has a finite number of combinations for unique IP addresses and that at some point, they will be over. This is why the IPv6 IP address was created. The IPv6 address is expected to launch worldwide in late 2010 or 2011, when the IPv4 addresses will reach their limit. The new address is of 128 bits (IPv4 is 32bit), which will be enough for at least 100 years in the future, even if the growth of the Internet users increases.

Dotted Decimal Notation of IP Addresses

32 bit binary number is hard to remember
Divide 32 bits into four 8-bit sections called octet
Express each octet as a decimal number between 0 to 255
Separate 4 decimal number by dot “.”
Easy for human and used by application programs

IP Addressing Principle for Router

Router connects to multiple physical networks
Each physical network has a unique prefix (Netid)
Router has multiple IP addresses, each for a connected network
Suffix in router IP address is assigned locally, like other host
Same suffix in routers’ IP addresses can help network administration