• To be able to identify a host on the internetwork, each host is assigned a unique address, the IP address.
• When a host has multiple network adapters, each adapter has a separate IP address.
• The IP address (sometimes called the INTERNET ADDRESS) consists of two parts:

• The network address part of the IP address is assigned by a central authority and is unique throughout the Internet. The central authority is the Internet Network Information Center (the InterNIC).
• The authority for assigning the host address part of the IP address resides with the organization which controls the network identified by the network number.
• IP addresses are 32-bit numbers usually represented in a dotted decimal form (as the decimal representation of four 8-bit values concatenated with dots).
• For example 128.2.7.9 is an IP address with 128.2 being the network number and 7.9 being the host number. The rules used to divide an IP address into its network and host parts are explained below.
• The binary format of the IP address 128.2.7.9 is:

• IP addresses are used by the IP protocol to uniquely identify a host on the Internet.
• IP datagrams (the basic data packets exchanged between hosts) are transmitted by some physical network attached to the host and each IP datagram contains a source IP address and a destination IP address.
• To send a datagram to a certain IP destination, the target IP address must be translated or mapped to a physical address.
• The five classes of IP addresses.
• Due to the explosive growth of the Internet, the use of assigned IP addresses became too inflexible to allow easy changes to local network configurations.
• This problem lead to the concept of subnetting.
• Whenever a host has a physical connection to multiple networks or subnets, it is described as being multi-homed.
• All routers are multi-homed since their purpose is to join networks or subnets. A multi-homed host always has different IP addresses associated with each network adapter, since each adapter is in a different subnet or network.
• These special IP addresses are used for specific purposes.

  ALL BITS ZERO (0): This address is used for "Bootstrapping" purposes, i.e. when a host is trying to determine its IP address from a remote server.

  ALL BITS ONE (1): This address is used for "Broadcasting" purposes, i.e. when a host wants to contact all the hosts/networks.

• Unicasting, Multicasting and Broadcasting:

  The majority of IP addresses refer to a single recipient: these are called unicast addresses.

  However, there are two special types of IP address, which are used for addressing multiple recipients: broadcast addresses and multicast addresses. These addresses are used for sending messages to multiple recipients.

  Any protocol, which is connectionless, may send broadcast or multicast messages as well as unicast messages.

  A protocol that is connection-oriented can only use unicast addresses because the connection exists between a specific pair of hosts.

  There are a number of addresses, which are used for IP broadcasting: all use the convention that "all-bits 1" indicates "all." Broadcast addresses are never valid as source addresses, only as destination addresses.

  Broadcasting has a major disadvantage: its lack of selectivity. If an IP datagram is broadcast to a subnet, every host on the subnet will receive it, and have to process it to determine whether the target protocol is active. If it is not, the IP datagram is discarded. Multicasting avoids this overhead by using groups of IP addresses.

• THE IP ADDRESS EXHAUSTION PROBLEM
  • The number of networks on the Internet has been approximately doubling annually for a number of years. However, the usage of the Class A, B and C networks differs greatly: nearly all of the new networks assigned in the late 1980s were Class B, and in 1990 it became apparent that if this trend continued, the last Class B network number would be assigned during 1994.
  • On the other hand, Class C networks were hardly being used.
  • A long term solution has been proposed but it has not been implemented yet! Its called Ipv6 (a 128 bit addressing method. We would have approx. 6x10²³ address per square metre on the surface of earth!! ).

1. A Guide to the TCP/IP Protocol Suite, Floyd Wilder, Artech House, 1993.

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