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Internet Protocol (IP)  | Tech Info | Network Information | TCP | TCP/IP |

General Description, Packet Format

General Information

The Internet protocol is a connectionless protocol that does NOT guarantee delivery of packets. Packets can be sent in smaller portions causing fragmentation and delivery of packets out of sequence. The receiving host can reassemble the packets using the information contained within the packets. Multiple routes can be taken to the destination. IP performs routing with the help of routing tables which are maintained by protocols such as RIP

IP is covered in RFC760 -

The IP model has 4 layers

IP characteristics

Note: IP addressing is capable of creating sub-networks from assigned addresses as described in RFC950

 

Format of an IP packet

Start bit Length Description Explanation
0 4 bits Version It contains the revision number of the protocol which must be agreed upon by both the sending node and the receiving node and the routers used for the connection.
4 4 bits IHL
IP Header Length		 It specifies the entire length of the datagram header, measured in 32-bit words. If the header does not use the Options field it is 5words long.
8 8 bits Type of Service Normally not used. When it is used, the upper-layer protocols tell IP how to handle a datagram as follows: P P P D T R x x where
Symbol position Description Setting
P 0-2 Precedence 0-7, Zero is normal and 7 is the highest priority
D 3 Delay 0-Normal, 1-Low
T 4 Throughput 0-Normal, 1-High
R 5 Reliability 0-Normal, 1-High
x 6-7 Reserved for future use set to zero for all packets
16 16 bits Total Length The size of the entire IP packet.
32 8 bits Identification Identification field used in reconstructing the packets in their original sequence
48 1 bit xF (flag) Reserved always zero
49 1 bit DF (flag) 0-May Fragment, 1-Do not fragment
50 1 bit MF (flag) 0-Last Fragment, 1-More Fragments
51 13 bits Fragment Offset The distance of this fragment's data from the start of the original datagram.
64 8 bits Time-to-Live (TTL) Value can range from 1 to 255 (4.25 minutes). Every time a datagram crosses a route the TTL value is decremented by 1 ( or more for slower routes). The datagram must reach the destination before this counter reaches zero other wise it is considered undeliverable.
72 8 bits Protocol Designates which upper layer protocol is to receive the packet. UDP has a value of 17 and TCP a value of 6.
80 16 bits Checksum Header A data integrity computation performed at each router (since TTL changes) on the entire header.
96 32 bits Source Address The sender's IP address in dotted decimal notation ( ie 127.0.1.1)
128 32 bits Destination Address The receiver's IP address in dotted decimal notation ( ie 127.0.1.1)
160 <= 320 bits Options & Padding Supports optional services like source route information, timestamps...
192 Variable Data-Variable Length  

Classes - 4 bytes as A.B.C.D

Class A - Always starts with zero (0) and varies up to 127. The first byte is the Network address and the last 3 bytes the Host address. It is capable of few Networks and many hosts.

Class B - Addresses start with 128 to 191. Bit pattern always starts with one, and then zero. The first two bytes are the network address, and the other two bytes are the host address.

Class C - Addresses start with 192 to 223. In binary form, they always start with 110. The first three bytes are the network addresses and the last byte is the host address.

Class D - Addresses start with 224 to 239. In binary this always starts with 1110 and mostly used for multicast packets.

Class E - They start with 240 to 255. In binary they always start with 1111 and are reserved for experimentation.

 

 

 

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