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freeCodeCamp/guide/english/network-engineering/ipv4-header/index.md

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IPv4 Header

IPv4 Header Format

Below is the IPv4 header format according to RFC791:

Image of IPv4 Header

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]
  • 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 of the one's complement sum of all 16-bit words in the header. [RFC792]
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

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