Datagram:

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Packets in the IPV4 layer are called datagrams .
A datagram is a variable length packet consisting of two parts. These are
• Header
• Data
The header is 20 to 60 bytes and contains information i.e essential for routing and delivery and the header is represented in 4-byte sections. These are
1. Version
2. Header length
3. Service
4. Total length

1. Version(VER):
This 4-bit field defines the version of the IPV4 protocol. Currently the version is 4. This field tells the IPv4 software running in the processing machine that the datagram has the format of version 4. All fields most be interpreted as specified in the fourth version of the protocol.

2. Header length(HLEN):
This 4-bit field defines the total length of the datagram header in 4-byte words. This field is needed because the length of the header is variable(between 20 and 60 bytes). When there are no options , the header length is 20 bytes and the value of this field is 5(5*4=20).

3. Service type:
In this interpretation, the first 3 bits are called precedence bits. The next 4 bits are called type of service(TOS) bits, and the last bit is not used.

4. Total length:
This is a 16-bit field that defines the total length (header plus data) of the IPv4 datagram in bytes. The total length field defines the total length of the datagram including the header.