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Appendix C     Packet Header Formats

Detailed information about the IP packet header can be found in RFC 791 which

is available from ftp://ftp.isi.edu/in notes/rfc791.txt and many other places including

the RFC editor web site. A brief explanation of different fields in the IP packet header is

found in Table C 1.

Table C 1 IP Packet Header Fields

Field

Explanation

V

Version number. The value is 4 for IPv4. Four bits are used for this part.

IHL

This field shows length of IP packet header. This is used to find out if the 

options part is present after the basic header. Four bits are used for IHL and it 

shows length in 32 bit word length. The value of this field for a basic 20 bytes 

header is 5.

TOS

This field shows type of service used for this packet. It is 8 bits in length.

Total Length

This field shows the length of the IP packet, including the data part. It is 16 bits 

long.

ID

This field packet identification number. This part is 16 bits long.

F

This part is three bits long and it shows different flags used in the IP header.

Frag Offset

This part is thirteen bits long and it shows fragment offset in case an IP packet 

is fragmented.

TTL

This is time to live value. It is eight bits long.

Protocol

This part shows transport layer protocol number. It is eight bits long.

Header Checksum

This part shows header checksum, which is used to detect any error in the IP 

header. This part is sixteen bits long.

Source Address

This is the 32 bit long source IP address.

Destination Address

This is the 32 bit long destination IP address.

ICMP Packet Header

ICMP header is completely explained in RFC 792, which is available from ftp://

ftp.isi.edu/in notes/rfc792.txt for download. Figure C 2 shows basic structure of ICMP

header. Note that depending upon type of ICMP packet, this basic header is followed by

different parts.