TCP/IP Network

Data Delivery

Two basic steps must be completed to deliver data successfully between two Internet participants.
First, it is necessary to transmit the data across the network to the appropriate host. Second, the
data has to be transmitted within that host to the appropriate user or process. TCP/IP uses threeschemes to accomplish these tasks:
1. Addressing
IP addresses uniquely identify each host on the entire internet; TCP/IP relies
on IP addressing to deliver data to the correct host on the network.
2. Routing
The Internet consists of many interconnected networks; different networks are
connected over routers (gateways). Routing means to forward data to the
correct network (or subnetwork) via an appropriate router (gateway).
3. Multiplexing
Protocol numbers and port numbers identify how to deliver data to the correct
software module within the host.

IP Address Classes
The Internet protocol (IP) moves data between hosts in the form of datagrams. Each datagram is delivered to the host identified by a 32−bit IP address located in the Destination Address field in the datagram header.
An IP address contains a network part and a host part, but the format of these parts is not uniformly determined; the number of address bits used to identify the network and the number used to identify the host vary according to the class of the address. The three address classes are class A, class B, and class C.
By examining the first few bits of an address, IP software can quickly determine the
address's class and, therefore, its structure.
Figure 15.1 illustrates how the address structure varies with an address class. It presents three address's classes in three arbitrary IP address examples.
The rules for address classes are:

• If the first address bit is 0, this is the address of a class A network; the next seven bitsidentify the network itself, and the remaining 24 bits identify the host in that network. Thereare fewer than 128 class A network addresses and 16 million hosts' addresses available ineach network (a certain number of network addresses are reserved for special purposes).
  
  
• If the first two bits are 10, this is the address of a class B network; the next 14 bits identifythe network, and the remaining 16 bits identify the host in that network. There are slightly more than 16,000 class B network addresses and more than 64,000 hosts' addresses available in each network.
  
  
• If the first two bits are 11, this is the address of a class C network; the next 22 bits identifythe network, and the last 8 bits identify the host in that network. There are more than 4 million class C network addresses with 256 hosts' addresses available (actually this number is lower, because the host's address 0 is reserved to identify the network itself, and the address 255 is the network broadcast address). The C class addresses that start with the first three bits 111 are reserved for special purposes, which means 2 million class C network addresses are available for general use.