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Unit 10: Network Layer in the Internet

without considering as to whether or not these machines are on the same network or not. The Notes
steps involved in communication in the Internet are given below:

 The transport layer immediately above the network layer breaks data streams into datagrams.
The size of the datagram size can be up to 64 KB but practically, it is around 1500 bytes so that
they can fit into an Ethernet frame. Each datagram or fragment is a packet.

 Each datagram is transmitted through the Internet. Large datagrams are non-transparently
fragmented, when required, into smaller units.
 When all the pieces reach to the destination machine, they are reassembled by the network
layer into the original datagram.
 The reassembled datagram is handed over to the transport layer, which inserts it into the
receiving process’ input data stream.
 IP is an unreliable connectionless protocol.

10.1 IP Protocol

In contrast to TCP, it is a connectionless type service and operates at third layer of OSI reference
model. That is, prior to transmission of data, no logical connection is needed. This type of
protocol is suitable for the sporadic transmission of data to a number of destinations. It does not
have such functions as sequence control, error recovery and control, flow control but it identifies
the connection with port number. The IP datagram has a header of 20-byte fixed size and a text
of variable length optional parts. The header format of IP datagram is depicted in Figure 10.1.
The header format is transmitted from left to right, with the high order bit of Version field is
transmitted first.
Figure 10.1: The IP (Internet Protocol) Header
32 Bits

Version IHL Types of service Total length
Identification DF MF Fragment Offset
Time to Live Protocol Header checksum
Source Addresses
Destination Addresses
Options (0 or more words)

Data encapsulation adds the IP header to the data. The IP header consists of five or six 32-bit
words; the sixth word is attributed to the IP options field. The different fields of the IP header are
given as below:
 Version refers to the version of the IP protocol in use and keeps track of the version of the
protocol to which the datagram belongs to. The current version of IP is 4.

 Internet Header Length (IHL) indicates the length of the header field in 32-bit words. The
minimum value of the header field is 5 that apply when no option is present. The maximum
value of this 4 bit filed is 15 that restricts the header to 60 bytes and thus Option field to 40
byte.

 Type of service enables the host to indicate the subnet what kind of service (e.g., reliability
and speed) it wants. It refers to any of the type of services that IP supports. Desired service

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