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Replaced stub for IPv4 header with image and descriptions for IPv4 header fields (#26655)

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Christopher McCormack
2018-10-26 12:18:06 -07:00
committed by Aditya
parent e97de4b4cb
commit 48077aa523
1 changed files with 52 additions and 6 deletions
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guide/english/network-engineering/ipv4-header/index.md
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@ -3,13 +3,59 @@ title: IPv4 Header
--- ---
## IPv4 Header Format ## IPv4 Header Format
This is a stub. <a href='https://github.com/freecodecamp/guides/tree/master/src/pages/network-engineering/ipv4-header/index.md' target='_blank' rel='nofollow'>Help our community expand it</a>. Below is the IPv4 header format according to [RFC791](https://tools.ietf.org/html/rfc791):
![Image of IPv4 Header](https://i.imgur.com/5ys9G3k.png)
##### Version [4-bits]
* Internet Protocol (IP) version number (decimal 4, binary 0100, hex 4)
##### Internet Header Length (IHL) [4-bits]
* Length of the IPv4 header
* The value of the IHL is measured in 32-bit words
* A value of 5 specifies the length of the header is 5 * 32-bits = 160bits = 20 bytes
* The minimum value of an IPv4 header is 20 bytes
##### Differentiated Services Code Point (DSCP) [6-bits]
* This value is used by network devices to determine precedence of an IP packet [[RFC2474](https://tools.ietf.org/html/rfc2474)]
* Rules can be created on network devices to handle different DSCP values appropriately
* For example - VoIP traffic may be given a DSCP value of 46 (binary 101110, hex 0x2E) which may inform a network device this packet should be given high priority
##### Explicit Congestion Notification (ECN) [2-bits]
* An optional feature that allows communicating end hosts to identify network congestion without necessarily dropping packets.
* ECN feature is only available if the underlying network infrastructure also supports it.
##### Total Length [16-bits]
* Length of the packet, including the header.
* This value is measured in bytes (8-bits), and has a maximum value of 2^16 - 1 = 65,535 bytes
##### Identification [16-bits]
* Assigned by the sending host to help in assembling the fragments of a packet
##### Flags [3-bits]
* Used to determine if a packet may be fragmented or to inform the receiving host that additional fragmented packets are inbound
* The Do Not Fragment (DF) bit is used when a packet may not be fragmented. If a device must fragment the packet because the packet is larger than the Maximum Transmission Unit (MTU) of the link, the packet is dropped
##### Fragment Offset [13-bits]
* The calculated offset for a fragmented packet relative to the beginning of the original unfragmented packet
* Measured in 8-byte blocks
* Maximum value is (2^13 - 1) * 8 = 65,528 bytes
##### Time To Live (TTL) [8-bits]
* The amount of time (in seconds) a packet is allowed to remain in the network
* This value is increased by one second at each network device encountered
* When the value reaches zero, the packet is discarded
##### Protocol [8-bits]
* Similar to the Next Header field in IPv6, this field specifies the protocol encapsulated in the IPv4 packet
* For example, TCP Protocol Number 6 would be binary 110, hex 0x6
##### Header Checksum [16-bits]
* The checksum is the 16-bit [one's complement](https://www.cs.uaf.edu/2004/fall/cs301/notes/node41.html) of the one's complement sum of all 16-bit words in the header. [[RFC792](https://tools.ietf.org/html/rfc791#page-14)]
##### Source IP Address [32-bits]
* The source IP address of the sender of the packet
##### Destination IP Address [32-bits]
* The destination IP address for the receiver of the packet
##### Options [32-bits]
* Specifies additional options
* Only present if the IHL is greater than 5, meaning the length is more than 5 * 32-bit words or 160-bits or 20 bytes - the length of a standard IPv4 header without options
* Options must include additional padding if they do not completely fill a 32-bit word boundary
<a href='https://github.com/freecodecamp/guides/blob/master/README.md' target='_blank' rel='nofollow'>This quick style guide will help ensure your pull request gets accepted</a>.
<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds -->
#### More Information: #### More Information:
<!-- Please add any articles you think might be helpful to read before writing the article --> * IP Option Numbers [[iana.org]](https://www.iana.org/assignments/ip-parameters/ip-parameters.xhtml)
* List of IP protocol numbers used in the Next Header [[iana.org]](https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml)