Internet Protocol Reference; Internet Protocol Header - Watchguard Firebox X20E Reference Manual

Internet Protocol Reference

CHAPTER 1
Internet Protocol (IP) specifies the format of packets and the addressing scheme for
sending data over the Internet. By itself, it functions like a postal system allowing you
to address a package and drop it into the system. There is, however, no direct link
between you and the recipient. In other words, there is no package.
Most networks combine IP with higher-level protocols like Transmission Control
Protocol (TCP). Unlike simple IP, TCP/IP establishes a connection between two host
servers so that they can send messages back and forth. TCP/IP provides the
"packaging."

Internet Protocol header

Internet Protocol (IP) is an Internet standard and enables the shipment of datagrams
– self-contained packets of information that include their own address and delivery
instructions. IP prepends a header to each datagram. The IP header contains a
minimum of twelve attributes as well as additional optional attributes.
Attribute
Version
IHL
TOS
Tot_Len
ID
Flags
Frag_Off
TTL
Reference Guide
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 generally
under-utilized because few application layers can set it.
16 bits Total length of packet measured in octets. It is used in
reassembling fragments.
16 bits Packet ID, used for reassembling 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.
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