IPv4

Internet Protocol version 4 (IPv4) is the fourth version in the development of the Internet Protocol (IP).

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IPv4 Header



 * Version (4 bits) - The Version field (4 bits) in all IPv4 packets contains the value 4. By comparison, the Version field in all IPv6 packets contains the value 6.
 * IHL (Internet Header Length) (4 bits) - indicates how long the IPv4 packet header is. If IHL value is 5, the IP header is 20 bytes (which is no options and minimum possible length). IHL value multipled by 4 is the length of the IP header.
 * TOS (Type of Service) field (8 bits) - used to implement a fairly simple QoS (Quality of Service).
 * Total Length field (16 bits) - Length of entire IP Packet (including IP header and IP Payload). Max packet size = 2^16 = 65,535 Bytes
 * Identification (Fragment ID) field (16 bits) - identifies which original packet this fragment was from, to help in reassembling the fragmented packet later. If IP packet is fragmented during the transmission, all the fragments contain same identification no. to identify original IP packet they belong to.
 * Flags (3 bits) - The first flag bit is reserved and must be zero. The second bit is the DF (Don’t Fragment) flag.  If DF is set, the packet cannot be fragmented. The third bit is the MF (More Fragments) flag. If MF is set, there are more fragments to come. Unfragmented packets have the MF flag set to zero.
 * Fragment Offset field (13 bits) - used in reassembly of fragmented packets.
 * Time To Live (TTL) field (8 bits) - to prevent packets from circling around indefinitely on a network with routing loops. The maximum TTL value is 255.
 * Protocol field (8 bits) - defines the next header, which is found immediately after the IPv4 Packet Header (actually the first part of the packet payload). http://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
 * Header Checksum field (16 bits) - to protect the header of IPv4 data packets against data corruption. http://www.thegeekstuff.com/2012/05/ip-header-checksum/
 * Source Address field (32 bits) - contains the IPv4 address of the sender.
 * Destination Address field (32 bits) - contains the IPv4 address of the recipient.
 * Options (0 to 40 bytes) - Very rarely used. For the details, see [//tools.ietf.org/html/rfc791 RFC 791] (IPv4).

Links

 * http://www.sixscape.com/joomla/sixscape/index.php/technical-backgrounders/tcp-ip/ip-the-internet-protocol/ipv4-internet-protocol-version-4/ipv4-packet-header
 * http://www.tutorialspoint.com/ipv4/ipv4_packet_structure.htm