10.3 IP Addresses within the IP Header 
10.3.2 Network layer fields
Instructor Note
  The purpose of this target indicator is that the student be able to explain, in detail, what comprises the IP datagram. Relate this datagram -- a Layer 3 PDU -- to the frame format diagrams that students studied when learning about Layer 2. This will make the concepts of headers and fields more plausible. Have the students pay particular attention to the source and destination IP addresses. Also point out that while the IP datagram looks complicated, all of this "overhead" information is necessary for routing and "best effort delivery" of packets. Also note that the total length in bytes of this "overhead" is typically a small fraction of the total length of the entire packet -- it is mostly carrying upper layer encapsulated data.

Call attention to the fact that this seemingly large Layer 3 PDU (datagram, packet) acts as "data" for the Layer 2 PDU (frames). That is, packets are encapsulated into frames.

This TI is related to CCNA Certification Exam Objectives #2, #29, and #36.
The Layer 3 packet/datagram becomes the Layer 2 data, which is then encapsulated into frames (as previously discussed). Similarly, the IP packet consists of the data from upper layers plus an IP header, which consists of:
  • version - indicates the version of IP currently used (4 bits)
  • IP header length (HLEN) - indicates the datagram header length in 32 bit words (4 bits)
  • type-of-service - specifies the level of importance that has been assigned by a particular upper-layer protocol (8 bits)
  • total length - specifies the length of the entire IP packet, including data and header, in bytes (16 bits)
  • identification - contains an integer that identifies the current datagram (16 bits)
  • flags - a 3-bit field in which the 2 low-order bits control fragmentation – one bit specifying whether the packet can be fragmented, and the second whether the packet is the last fragment in a series of fragmented packets (3 bits)
  • fragment offset - the field that is used to help piece together datagram fragments (13 bits)
  • time-to-live - maintains a counter that gradually decreases, by increments, to zero, at which point the datagram is discarded, keeping the packets from looping endlessly (8 bits)
  • protocol - indicates which upper-layer protocol receives incoming packets after IP processing has been completed (8 bits)
  • header checksum - helps ensure IP header integrity (16 bits)
  • source address - specifies the sending node (32 bits)
  • destination address - specifies the receiving node (32 bits)
  • options - allows IP to support various options, such as security (variable length)
  • data - contains upper-layer information (variable length, maximum 64 Kb)

  • padding - extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits

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IP Addressing Architecture