Ans : IP is the transmission mechanism used by TCP/IP protocols for host-to-host
communication Packets in IP layer are called datagrams. Figure 5 shows the IP
datagram format:
A brief description of header fields in order is given below:
• Version (4 bits): It defines the version of IP protocol. Currently, the version is
4(IPv4), indicated by value 4. In future it would contain 6, for IPv6.
• HLEN (4 bits): It is needed because length of header is variable. When the
header size is 20 bytes, its value is 5(54=20). With options, the maximum size is 60 bytes, when the value is 15 (154=60). Each value represents number of 32-bit words.
• Service Type (8 bits): It is used to define type of service in terms of reliability,
precedence, delay and cost.
• Total length (16 bits): it defines the total length of IP datagram. The maximum
value can be 216=65536 bytes.
• Identification (16 bits): This field is used to unusually identify a datagram. It is
useful to know the fragments belonging to same datagram fragments that are part
of a datagram which contain same value in identification fields, so that they can
be put together in the order to reassemble the datagram at receiver.
• Flags (3 bits): This field is used to uniquely identify a datagram. It is useful to
know the fragments belonging to same datagrams.
• Fragmentation Offset (13 bits): It is a pointer that indicates the offset of the
fragment in the original datagram before fragmentation.
• Time to Live (8 bits): It is used to control the maximum number of hops visited
by the datagram. It is needed to restrict a datagram from continuing to travel in
infinite loop without reaching the destination. This infinite looping may cause
network congestion. This field limits the lifetime of datagram, after which the
packet is discarded, so that datagram does not travel in infinite loop.
• Protocol (8 bits): An IP datagram may encapsulate data from various higher-level protocols like TCP, UDP, ICMP, and IGMP. This field specifies the final destination protocol to which the IP datagram should be delivered. Each protocol
TCP, UDP etc. identified with a unique number.
• Source Address (32 bits): It stores the IP address of the source.
• Destination Address (32 bits): It stores the IP address of the final destination.
• Options: This field contains optional information such as routing details,
timestamp etc. For instance, it can store route of a datagram, in the form of IP
addresses of intermediate routers, optionally the time when it pass through that
router.
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