Information Technology Reference
In-Depth Information
Ta b l e 8 - 2
IP Protocol Numbers
Protocol Number
Protocol
89
Open Shortest Path First (OSPF)
103
Protocol-Independent Multicast (PIM)
112
Virtual Router Redundancy Protocol (VRRP)
Header Checksum:
This field is 16 bits in length. The checksum does not include
the data portion of the packet in the calculation. The checksum is recomputed and
verified at each point the IP header is processed.
■
Source Address:
This field is 32 bits in length. It is the sender's IP address.
■
Destination Address:
This field is 32 bits in length. It is the receiver's IP address.
■
IP Options:
This field is variable in length. The options provide for control functions
that are useful in some situations but unnecessary for the most common communica-
tions. Specific options are security, loose source routing, strict source routing, record
route, and timestamp.
■
Padding:
This field is variable in length. It ensures that the IP header ends on a 32-bit
boundary.
■
Ta ble 8 - 3 summarizes the fields of the IP header.
Ta b l e 8 - 3
IPv4 Header Fields
Field
Length
Description
Ve r s i o n
4 bits
Indicates the IP header's format, based on the version number. Set
to 0100 for IPv4.
IHL
4 bits
Length of the header in 32-bit words.
To S
8 bits
QoS parameters.
To t a l L e n g t h
16 bits
Length of the packet in bytes, including header and data.
Identification
16 bits
Identifies a fragment.
Flags
3 bits
Indicates whether a packet is fragmented and whether more frag-
ments follow.
Fragment
Offset
13 bits
Location of the fragment in the total packet.
Time to Live
8 bits
Decremented by 1 by each router. When this is 0, the router dis-
cards the packet.
Protocol
8 bits
Indicates the upper-layer protocol.
Header Check-
sum
16 bits
Checksum of the IP header; does not include the data portion.
