iii- Delivering Data through the Internet
To deliver data between two Internet hosts, it is necessary to move data across
the network to the correct host and within that host to the correct application.
TCP/IP accomplishes this job using concepts on addressing routing and
multiplexing. These three concepts in TCP/IP are necessary to deliver data from
one application to another across the network.
iv- ADDRESSING
The IP moves data between hosts in the form of datagrams. Earlier, we
illustrated that the datagram header contains the destination address. The
destination address is a standard 32-bit IP address that contains information to
uniquely a network and a specific host in that network.
An IP address caries information on the network and the host in that network.
However, the format used to identify the network and host is network and the
number of bits used to identify the host varies according to the class of the IP
address. The first few bits of an address indicate the class of the address. The
classes of IP addresses are:
1. Class A- The first bit of the IP address is O. the next seven bits identify
the network and the last 24 bits identify the host. There fewer than 128
class A network numbers, but each class A network may be composed of
millions of hosts.
2. Class B- The first two bits of the IP address are 10 to. The next 14 bits
identify the network and the last 16 bits identify the host. There are
thousands of class B network numbers where each class can contain
thousands of hosts.
3. Class C- The first three bits of the IP address are 110. the next 21 bits are
the network address and the last eight bits identify the host. There are
millions of class C networks but each class is composed of fewer than 254
hosts.
4. Class D- The first three bits of the IP address are 111. These addresses do
not really refer to networks but are used as multicast addresses. Multicast
addresses are used to address groups of computers all at once.
IP addresses are written as four –digit decimals separated by dots or periods,
e.g, 202.138.162.129. Each decimal number is in the range 0 to 225 (8 bits).
Hence, we actually have a sequence of 4 8-bits bytes in an IP address. The first
byte value indicates the class of the IP address. If the first byte value is.
1. less than 128, then this is a class A address. The first number in the
sequence indicates the network address; the next three numbers indicate
the host address. For example, the address 65.123.45.18 is interpreted as
host 123.45.18 on network 65.
2. from 128 to 191, then this is a class B address. The first two numbers in
the sequence indicate the network address and the next two number
indicate the host address. For example, the address 165.123.56.2 is
interpreted as host 56.2 on network 165.123.
3. from 192 to 223, then this is a class C address. The first three numbers in
the sequence comprise the network address and the last number is the
host address. For example, the address 202.138.162.129 is interpreted as
host 129 in the network 202.138.162.
4. greater than 223, then this is reserved and can just be ignored.
There are other IP addresses that are reserved aside from those where the
first number is greater than 223. For class A addresses, the addresses 0 and
127 are reserved. The address 0 and 127 are reserved. The address 0 is used
as a default route and 127 is the loopback address. Other reserved
addresses are host number 0 and 255. the IP address with host bits set to
zero identifies the network itself. For example, the address 11.0.0.0 refers to
network 11 and the address 130.77.0.0 refers to network 130.77.Another
reserved address is the IP address with the host bits all sets to 1, e.g.,
65.255.255.255. this is a aboard-cast maddress used to simultaneously
address every host in network 65.
One critical problem facing the implementation of the internet at present is
the problem of address depletion. With the exponential increase in number
of computer connected to the internet, it will not take long before the class A,
B and C addresses run out. One temporary solution being encouraged is to
form subnets from main networks. The standard structure of the IP address
can be locally bits. Essential, the dividing line between the network address
bits and host address bits is moved to the right, creating additional network
but reducing the maximum number of host that can belong within the larger
network, called a subnet.
A subnet is defined by applying a bit mask, the subnet mask
, to the IP address. If a bit is 1 on the mask, that equivalent bit in the address is
interpreted as a network bit. If a bit is 0 in the mask, the bit belongs to the
host part of the address. The subnet is known only locally. To the internet,
the address is still interpreted as standard IP address. For example, consider
the class B address 165.220.20.11. In this class B address, 165.220 comprise
the network bits 20.11 comprise the host bits. Using the subnet mask
255.255.255.0, the network portion can be extended by another byte. Hence,
165.220.20.11 IP address and subnet mask 255.255.255.0, refer to subnet
165.220.20 and host 11.
