internet module to another through individual networks based on the
  interpretation of an internet address.  Thus, one important mechanism
  of the internet protocol is the internet address.

  In the routing of messages from one internet module to another,
  datagrams may need to traverse a network whose maximum packet size is
  smaller than the size of the datagram.  To overcome this difficulty, a
  fragmentation mechanism is provided in the internet protocol.

  Addressing

    A distinction is made between names, addresses, and routes [4].   A
    name indicates what we seek.  An address indicates where it is.  A
    route indicates how to get there.  The internet protocol deals
    primarily with addresses.  It is the task of higher level (i.e.,
    host-to-host or application) protocols to make the mapping from
    names to addresses.   The internet module maps internet addresses to
    local net addresses.  It is the task of lower level (i.e., local net
    or gateways) procedures to make the mapping from local net addresses
    to routes.

    Addresses are fixed length of four octets (32 bits).  An address
    begins with a network number, followed by local address (called the
    "rest" field).  There are three formats or classes of internet
    addresses:  in class a, the high order bit is zero, the next 7 bits
    are the network, and the last 24 bits are the local address; in
    class b, the high order two bits are one-zero, the next 14 bits are
    the network and the last 16 bits are the local address; in class c,
    the high order three bits are one-one-zero, the next 21 bits are the
    network and the last 8 bits are the local address.

    Care must be taken in mapping internet addresses to local net
    addresses; a single physical host must be able to act as if it were
    several distinct hosts to the extent of using several distinct
    internet addresses.  Some hosts will also have several physical
    interfaces (multi-homing).

    That is, provision must be made for a host to have several physical
    interfaces to the network with each having several logical internet
    addresses.




 

                                                          September 1981
Internet Protocol
Overview



    Examples of address mappings may be found in "Address Mappings" [5].

  Fragmentation

    Fragmentation of an internet datagram is necessary when it
    originates in a local net that allows a large packet size and must
    traverse a local net that limits packets to a smaller size to reach
    its destination.

    An internet datagram can be marked "don't fragment."  Any internet
    datagram so marked is not to be internet fragmented under any
    circumstances.  If internet datagram marked don't fragment cannot be
    delivered to its destination without fragmenting it, it is to be
    discarded instead.

    Fragmentation, transmission and reassembly across a local network
    which is invisible to the internet protocol module is called
    intranet fragmentation and may be used [6].

    The internet fragmentation and reassembly procedure needs to be able
    to break a datagram into an almost arbitrary number of pieces that
    can be later reassembled.  The receiver of the fragments uses the
    identification field to ensure that fragments of different datagrams
    are not mixed.  The fragment offset field tells the receiver the
    position of a fragment in the original datagram.  The fragment
    offset and length determine the portion of the original datagram
    covered by this fragment.  The more-fragments flag indicates (by
    being reset) the last fragment.  These fields provide sufficient
    information to reassemble datagrams.

    The identification field is used to distinguish the fragments of one
    datagram from those of another.  The originating protocol module of
    an internet datagram sets the identification field to a value that
    must be unique for that source-destination pair and protocol for the
    time the datagram will be active in the internet system.  The
    originating protocol module of a complete datagram sets the
    more-fragments flag to zero and the fragment offset to zero.

    To fragment a long internet datagram, an internet protocol module
    (for example, in a gateway), creates two new internet datagrams and
    copies the contents of the internet header fields from the long
    datagram into both new internet headers.  The data of the long
    datagram is divided into two portions on a 8 octet (64 bit) boundary
    (the second portion might not be an integral multiple of 8 octets,
    but the first must be).  Call the number of 8 octet blocks in the
    first portion NFB (for Number of Fragment Blocks).  The first
    portion of the data is placed in the first new internet datagram,
    and the total length field is set to the length of the first