IPv4 Packet Format

Figure 1 shows the IPv4 packet format.

Figure 1 IPv4 packet format

An IPv4 datagram consists of a header and a data field. The first 20 bytes in the header are mandatory for all IPv4 datagrams. The Options field following the 20 bytes has a variable length.

Table 1 describes the meaning of each field in an IPv4 packet.

**Table 1** Description of each field in an IPv4 packet
Field	Length	Description
Version	4 bits	Specifies the IP protocol version, IPv4 or IPv6.
Header Length	4 bits	Specifies the length of the IPv4 header.
Type of Service (ToS)	8 bits	Specifies the type of service. This field takes effect only in the differentiated service model.
Total Length	16 bits	Specifies the length of the header and data.
Identification	16 bits	IPv4 software maintains a counter in the storage device to record the number of IP datagrams. The counter value increases by 1 every time a datagram is sent, and is filled in the identification field.
Flags	3 bits	Only the rightmost two bits are valid. The rightmost bit indicates whether the datagram is not the last data fragment. The value 1 indicates the last fragment, and the value 0 indicates non-last fragment. The middle bit is the fragmentation flag. The value 1 indicates that the datagram cannot be fragmented, and the value 0 indicates that the datagram can be fragmented.
Fragment Offset	13 bits	Specifies the location of a fragment in a packet.
Time to Live (TTL)	8 bits	Specifies the life span of a datagram on a network. TTL is measured by the number of hops.
Protocol	8 bits	Specifies the type of the protocol carried in the datagram.
Header Checksum	16 bits	A device calculates the header checksum for each datagram received. If the checksum is 0, the device knows that the header remains unchanged and retains the datagram. This field checks only the header but not the data.
Source IP Address	32 bits	Specifies the IPv4 address of a sender.
Destination IP Address	32 bits	Specifies the IPv4 address of a receiver.
Options	0-40 bytes (variable length)	Allows IPv4 to support various options such as fault handling, measurement, and security. Pad bytes with a value of 0 are added if necessary.
Data	Variable	Pads an IP datagram.