Internet Protocol Header - Watchguard Firebox X5500E Reference Manual

Internet Protocol Header

Internet Protocol (IP) is an Internet standard that enables the sending of datagrams — packets of
information that include an address and instructions on how to send the datagram to its destination.
IP prepends a header to each datagram. The IP header contains a minimum of 12 properties, and other
optional properties.
Property
Version
IHL
TOS
Tot_Len
ID
Flags
Frag_Off
TTL
Protocol
Check
Sour_Addr
Dest_Addr
Options
2
Size Description
4 bits IP format number (Current version = 4)
4 bits Header length in 32-bit words (Minimum = 5)
8 bits Type of service sets routing priorities. It is usually not used because not many
application layers can set it.
16 bits Total length of packet measured in octets. It is used to assemble fragments.
16 bits Packet ID, used to assemble fragments.
3 bits Miscellaneous flags
13 bits Identifies fragment part for this packet.
8 bits Time to live. It sets the maximum time the datagram remains alive in the system.
8 bits IP protocol number. Indicates which of TCP, UDP, ICMP, IGMP, or other Transport
protocol is inside.
16 bits Checksum for the IP header
32 bits Source IP address
32 bits Destination IP address
24 bits IP Options (Present if IHL is 6)
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