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54f. IP Header |
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54f. IP Header
The IP datagram is traditional represented by many rows of 32 bits (4 octets or bytes). Each 32 bit word is stacked on top of each other as per the following diagram for the IP header:
Field Description
Version (4 bits) The IP version, currently it is version 4
0 Reserved
1-3 Unassigned
4 IP Internet Protocol
5 ST ST Datagram Mode
6 SIP Simple Internet Protocol
7 TP/IX TP/IX: The Next Internet
8 PIP The P Internet Protocol
9 TUBA TUBA
10-14 Unassigned
15 Reserved
IHL (4 bits) Internet Header Length, the length of the IP header in 32 bit words.
Type of Service Flags to indicate precedence, delay, throughput and
(8 bits) reliability parameters.
Bit 0-2: Precedence
000 - Routine
001 - Priority
010 - Immediate
011 - Flash
100 - Flash override
101 - CRITIC / ECP
110 - Internet control
111 - Network control
Bit 3: 0 = Normal delay, 1 = Low delay
Bit 4: 0 = Normal throughput, 1 = High throughput
Bit 5: 0 = Normal reliability, 1 = High reliability
Bit 6-7: Reserved for future use.
Total Length Total length in bytes of the IP datagram (IP header and data)
(16 bits) Minimum length = 576, Maximum length = 65,535
Identification Unique identifying number for this datagram
(16 bits)
Flags (3 bits) Options that indicate if fragmentation s permitted and/or used
Bit 0: Reserved, allways set to 0
Bit 1: 0 = May fragment, 1 = Don't fragment
Bit 2: 0 = Last fragment, 1 = More fragments
Fragment Offset Indicates where in the entire datagram, this particular fragment
(13 bits) belongs. Measured in 64 bit units from the beginning of the
initial datagram.
Time to live (TTL) Measured in hop counts or seconds. Every transverse through
(8 bits) a router or gateway will decrement the hop count. When the
TTL equals 0, the datagram is discarded. This stops datagrams
from circulating the network forever. Starts at TTL = 255.
Protocol Identifies the next protocol that follows the IP header.
(8 bits) The full listing of protocols is in
Appendix G: IP Header Protocols.
Examples are:
Decimal Protocol Description
0 Reserved
1 ICMP Internet Control Message
2 IGMP Internet Group Management
6 TCP Transmission Control
37 DDP Datagram Delivery Protocol
46 RSVP Reservation Protocol
93 AX.25 AX.25 Frames
Header Checksum A 32 bit Cyclic Redundacy Check that may be checked at each
(32 bit) gateway.
Source Address An IP address indicating the sender. Ex. 142.110.237.1
(32 bit)
Destination Address An IP address indicating the receiptient. Ex. 142.110.237.2
(32 bit)
Options Options from the sender such as route specifications.
(variable)
Padding Ensures that the IP header ends on a 32 bit boundary.