Anthony Fajri

IP Header

This topic is a very old story. But if you work in Networking area, you should know this story. May be it’s a nice tale before sleep to my son.

Long time ago, when I was still not born yet , some people outhere made an internet standard for something called Internet Protocol. This standard were documented in RFC 791. Because of this tale, you can read my blog (if you read my blog).

One of the important thing about this document is IP Header. IP Header is something look like this picture :

Look, this is the sample of IP header, captured by ethereal

Let me tell you about every block :

Ver is a 4 bits value represent the version of internet protocol. The value depends on what version of internet protocol we are using, for example: 4 for IPv4, 6 to IPv6, 16 for Novell IPX (RFC 1639)

IHL (stands for Internet Protocol Header Length) is a 4 bits value to represent the length of internet header in 32 bit words

Type of Services is a 8 bit value to represent the quality of service desired. The value of each bits are as shown in the picture below :

Total Length is a 16 bit value represent the total length (in bytes) of the datagram including the header and the data.

Identification is a 16 bit identification value used to reassembly the fragmented packet.

Flags is a 3 bit value to control the fragmented datagram. The values are shown in this picture.

Fragment Offset is a 13 bit value to represent the location of fragmented datagram in the datagram count from the start of the original sent packet.

TTL (stands for Time To Live) is a 8 bit value to represent the number of hop the datagram may be routed. Once the datagram reach one hop, the value is decreased by one. If the value is zero, then the datagram is destroyed.

Protocol is 8 bit value to represent the protocol used by the datagram. For example 1 for ICMP, 6 for TCP, 17 for UDP, 50 for ESP. The list of the protocol and its value is provided by IANA in this link.

Header Checksum is 16 bit value to represent the 1’s complement checksum inserted by the original sender and updated by each hop the datagram was passing. The function of this checksum header is to detect the processing error in the datagram.

Source IP and Destination IP are 32 bits value to represent the source IP and destination IP. Then, what about IPv6 which is 128 bits? a 32 bits space for source IP and destination IP will not enough for IPv6.

Then thanks to some geeks overthere making this stuff possible. They told this things in another story and another RFC. It’s RFC 2640. Yes, their story simplified the IPv6 header. They do manything to simplify the header such as no header checksum, no fragmentation in the router, fixed length of header, etc..

These new things are … ops.. Saladin’s already sleeping. Okay son, I’ll tell you about IPv6 header in the next story. Have a nice dream. *kissing my son’s forehead* I love you.

February 3rd, 2007 Posted by Anthony Fajri | information technology | 5 comments