•  Identifies the IP version to which the packet belongs.
• 4 bit field is usually set to binary 0100. Version 4 (IPv4) is in current, common use.

• is a four-bit field that tells the length of the IP header.
• The minimum length of the IP header is 20 octets, and the options may increase this size   up to a  maximum of 24 octets.
• This field describes the length of the header in terms of 32-bit words— five for the minimum
• 160-bit size and six for the maximum.

•  is an eight-bit field that can be used for specifying special handling of the packet.
• This field actually can be broken down into two subfields: Precedence and TOS.
•  Precedence sets a priority for the packet, the way a package might be sent overnight, 2-day      delivery, or general post.
• TOSallows the selection of a delivery service in terms of throughput, delay, reliability, and       monetary cost.

• is a 16-bit field specifying the total length of the packet, including the header, in octets.
• By subtracting the header length, a receiver may determine the size of the packet’s data payload.
• Because the largest decimal number that can be described with 16 bits is 65,535, the maximum  possible size of an IP packet is 65,535 octets.

•   is a 16-bit field used in conjunction with the Flags and Fragment Offset fields for fragmentation of a packet.
• Packets must be fragmented into smaller packets if the original length exceeds the Maximum Transmission Unit (MTU) of a data link through which they pass.
• For example, consider a 5,000-byte packet traveling through an internetwork.
• It encounters a data link whose MTU is 1,500 bytes— that is, the frame can contain a maximum packet size of 1,500 bytes.
• The router that places the packet onto this data link must first fragment the packet into chunks of no more than 1,500 octets each.
• The router then marks each fragment with the same number in the Identifier field so that a receiving device can identify the fragments that go together.

•   is a three-bit field in which the first bit is unused. 
•  The second is the Don’t Fragment (DF) bit. 
•  When the DF bit is set to one, a router cannot fragment the packet. 
•  If the packet cannot be forwarded without fragmenting, the router drops the packet and sends an  error message to the source. 
•  This function enables the testing of MTUs in an internetwork. 
•  The DF bit can be set using the Extended Ping utility on Cisco routers. 
•  The third bit is the More Fragments (MF) bit. 
•  When a router fragments a packet, it sets the MF bit to one in all but the last fragment so that the receiver knows to keep expecting fragments until it encounters a fragment with MF = 0.

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•  is a 13-bit field that specifies the offset, in units of eight octets, from the beginning of the header to  the beginning of the fragment. 
•  Because fragments may not always arrive in sequence, the Fragment Offset field allows the pieces  to be reassembled in the correct order. 
•  If a single fragment is lost during a transmission, the entire packet must be resent and refragmented at the same point in the internetwork.

•   is an eight-bit field that will be set with a certain number when the packet is first generated. 
•  As the packet is passed from router to router, each router will decrement this number. 
• If the number reaches zero, the packet will be discarded and an error message will be sent to the  source. 
•  This process prevents “lost” packets from wandering endlessly through an internetwork. 
• The TTL was specified in seconds; if a packet was delayed more than a second in a router, the   router would adjust the TTL accordingly. 
•  However, this approach is difficult to implement and is rarely supported.–   Most routers simply decrement the TTL by one, no matter what the actual delay, so the TTL is really a hop count. 
• The recommended default TTL is
•  64, although values such as 15 and 32 are not uncommon.

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•   make use of the TTL field. 
•  If the router is told to trace the route to a host address such as 10.11.12.13, the router will send  three packets with the TTL set to one; the first router will decrement it to zero, drop the packets, and send back error messages to the source. 
• By reading the source address of the error messages, the first router on the path is now known. 
• The next three packets will be sent with a TTL of two. 
• The first router decrements to one, the second to zero, and an error message is received from the second router. 
•  The third set has a TTL of three, and so forth, until the destination is found. All routers along the internetwork path will have identified themselves.

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• is an eight-bit field that gives the “address,” or protocol number, of the host-to-host or transport layer protocol for which the information in the packet is destined.

•  is the error correction field for the IP -header. 
•  The checksum is not calculated for the encapsulated data; UDP, TCP, and ICMP have their own checksums for doing this. 
•  The field contains a 16-bit one’s complement checksum, calculated by the originator of the packet. 
• The receiver will again calculate a 16-bit one’s complement sum, including the original checksum. 
•  If no errors have occurred during the packet’s travels, the resulting checksum will be all ones. 
•  Remember that each router decrements the TTL; therefore, the checksum must be recalculated at each router.

•  are the 32-bit IP addresses of the originator of the packet and the destination of the packet.

• is a variable-length field and, as the name implies, is optional. – Space is added to the packet header to contain either source-generated information or for other routers to enter information; the options are used primarily for testing.  

•  ensures that the header ends on a 32-bit boundary by adding zeros after the option field      until a multiple of 32 is reached

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Disclaimer: The materials used for this post is for personal notes on my preparations for the CCIE certifications. Source of notes were taken from Routing TCP/IP Volume 1.