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IPV4 - PACKET STRUCTURE
Internet Protocolbeing a layer-3 protocol(OSI) takes data Segments fromlayer-4 (Transport) and divides it
into what’s called packet. IP packet encapsulates data unit received fromabove layer and adds its ownheader
information.
The encapsulated data is referred to as IP Payload. IP header contains allthe necessary informationto deliver
the packet at the other end.
IP header includes many relevant informationincluding VersionNumber, which, inthis context, is 4. Other details
are as follows:
Version: Versionno. of Internet Protocolused (e.g. IPv4)
IHL: Internet Header Length, Lengthof entire IP header
DSCP: Differentiated Services Code Point, This is Type of Service.
ECN: Explicit CongestionNotification, carries informationabout the congestionseeninthe route.
Total Length: Lengthof entire IP Packet (including IP header and IP Payload)
Identification: If IP packet is fragmented during the transmission, allthe fragments containsame
identificationno. to identify originalIP packet they belong to.
Flags: As required by the network resources, if IP Packet is too large to handle these ‘flags’ tellthat if
they canbe fragmented or not. Inthis 3-bit flag, the MSB is always set to ‘0’.
Fragment Offset: This offset tells the exact positionof the fragment inthe originalIP Packet.

2. Time to Live: To avoid looping inthe network, every packet is sent withsome TTL value set, whichtells
the network how many routers (hops) this packet cancross. At eachhop, its value is decremented by one
and whenthe value reaches zero, the packet is discarded.
Protocol: Tells the Network layer at the destinationhost, to whichProtocolthis packet belongs to, i.e.
the next levelProtocol. For example protocolnumber of ICMP is 1, TCP is 6 and UDP is 17.
Header Checksum: This field is used to keep checksumvalue of entire header whichis thenused to
check if the packet is received error-free.
Source Address: 32-bit address of the Sender (or source) of the packet.
Destination Address: 32-bit address of the Receiver (or destination) of the packet.
Options: This is optionalfield, whichis used if the value of IHL is greater than5. These optionmay
containvalues for options suchas Security, Record Route, Time Stamp etc.