rfc1180.txt

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   Each computer has a separate ARP table for each of its Ethernet
   interfaces.  If the target computer does not exist, there will be no
   ARP response and no entry in the ARP table.  IP will discard outgoing
   IP packets sent to that address.  The upper layer protocols can't
   tell the difference between a broken Ethernet and the absence of a
   computer with the target IP address.

   Some implementations of IP and ARP don't queue the IP packet while
   waiting for the ARP response.  Instead the IP packet is discarded and
   the recovery from the IP packet loss is left to the TCP module or the
   UDP network application.  This recovery is performed by time-out and
   retransmission.  The retransmitted message is successfully sent out
   onto the network because the first copy of the message has already
   caused the ARP table to be filled.

5.  Internet Protocol

   The IP module is central to internet technology and the essence of IP
   is its route table.  IP uses this in-memory table to make all
   decisions about routing an IP packet.  The content of the route table
   is defined by the network administrator.  Mistakes block
   communication.

   To understand how a route table is used is to understand
   internetworking.  This understanding is necessary for the successful
   administration and maintenance of an IP network.

   The route table is best understood by first having an overview of
   routing, then learning about IP network addresses, and then looking
   at the details.

5.1  Direct Routing

   The figure below is of a tiny internet with 3 computers: A, B, and C.
   Each computer has the same TCP/IP protocol stack as in Figure 1.
   Each computer's Ethernet interface has its own Ethernet address.
   Each computer has an IP address assigned to the IP interface by the
   network manager, who also has assigned an IP network number to the
   Ethernet.



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                          A      B      C
                          |      |      |
                        --o------o------o--
                        Ethernet 1
                        IP network "development"

                       Figure 6.  One IP Network

   When A sends an IP packet to B, the IP header contains A's IP address
   as the source IP address, and the Ethernet header contains A's
   Ethernet address as the source Ethernet address.  Also, the IP header
   contains B's IP address as the destination IP address and the
   Ethernet header contains B's Ethernet address as the destination
   Ethernet address.

                ----------------------------------------
                |address            source  destination|
                ----------------------------------------
                |IP header          A       B          |
                |Ethernet header    A       B          |
                ----------------------------------------
       TABLE 5.  Addresses in an Ethernet frame for an IP packet
                              from A to B

   For this simple case, IP is overhead because the IP adds little to
   the service offered by Ethernet.  However, IP does add cost: the
   extra CPU processing and network bandwidth to generate, transmit, and
   parse the IP header.

   When B's IP module receives the IP packet from A, it checks the
   destination IP address against its own, looking for a match, then it
   passes the datagram to the upper-level protocol.

   This communication between A and B uses direct routing.

5.2  Indirect Routing

   The figure below is a more realistic view of an internet.  It is
   composed of 3 Ethernets and 3 IP networks connected by an IP-router
   called computer D.  Each IP network has 4 computers; each computer
   has its own IP address and Ethernet address.










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          A      B      C      ----D----      E      F      G
          |      |      |      |   |   |      |      |      |
        --o------o------o------o-  |  -o------o------o------o--
        Ethernet 1                 |  Ethernet 2
        IP network "development"   |  IP network "accounting"
                                   |
                                   |
                                   |     H      I      J
                                   |     |      |      |
                                 --o-----o------o------o--
                                  Ethernet 3
                                  IP network "factory"

               Figure 7.  Three IP Networks; One internet

   Except for computer D, each computer has a TCP/IP protocol stack like
   that in Figure 1.  Computer D is the IP-router; it is connected to
   all 3 networks and therefore has 3 IP addresses and 3 Ethernet
   addresses.  Computer D has a TCP/IP protocol stack similar to that in
   Figure 3, except that it has 3 ARP modules and 3 Ethernet drivers
   instead of 2.  Please note that computer D has only one IP module.

   The network manager has assigned a unique number, called an IP
   network number, to each of the Ethernets.  The IP network numbers are
   not shown in this diagram, just the network names.

   When computer A sends an IP packet to computer B, the process is
   identical to the single network example above.  Any communication
   between computers located on a single IP network matches the direct
   routing example discussed previously.

   When computer D and A communicate, it is direct communication.  When
   computer D and E communicate, it is direct communication.  When
   computer D and H communicate, it is direct communication.  This is
   because each of these pairs of computers is on the same IP network.

   However, when computer A communicates with a computer on the far side
   of the IP-router, communication is no longer direct.  A must use D to
   forward the IP packet to the next IP network.  This communication is
   called "indirect".

   This routing of IP packets is done by IP modules and happens
   transparently to TCP, UDP, and the network applications.

   If A sends an IP packet to E, the source IP address and the source
   Ethernet address are A's.  The destination IP address is E's, but
   because A's IP module sends the IP packet to D for forwarding, the
   destination Ethernet address is D's.



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                ----------------------------------------
                |address            source  destination|
                ----------------------------------------
                |IP header          A       E          |
                |Ethernet header    A       D          |
                ----------------------------------------
       TABLE 6.  Addresses in an Ethernet frame for an IP packet
                         from A to E (before D)

   D's IP module receives the IP packet and upon examining the
   destination IP address, says "This is not my IP address," and sends
   the IP packet directly to E.

                ----------------------------------------
                |address            source  destination|
                ----------------------------------------
                |IP header          A       E          |
                |Ethernet header    D       E          |
                ----------------------------------------
       TABLE 7.  Addresses in an Ethernet frame for an IP packet
                         from A to E (after D)

   In summary, for direct communication, both the source IP address and
   the source Ethernet address is the sender's, and the destination IP
   address and the destination Ethernet address is the recipient's.  For
   indirect communication, the IP address and Ethernet addresses do not
   pair up in this way.

   This example internet is a very simple one.  Real networks are often
   complicated by many factors, resulting in multiple IP-routers and
   several types of physical networks.  This example internet might have
   come about because the network manager wanted to split a large
   Ethernet in order to localize Ethernet broadcast traffic.

5.3  IP Module Routing Rules

   This overview of routing has shown what happens, but not how it
   happens.  Now let's examine the rules, or algorithm, used by the IP
   module.

     For an outgoing IP packet, entering IP from an upper layer, IP must
     decide whether to send the IP packet directly or indirectly, and IP
     must choose a lower network interface.  These choices are made by
     consulting the route table.

     For an incoming IP packet, entering IP from a lower interface, IP
     must decide whether to forward the IP packet or pass it to an upper
     layer.  If the IP packet is being forwarded, it is treated as an



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     outgoing IP packet.

     When an incoming IP packet arrives it is never forwarded back out
     through the same network interface.

   These decisions are made before the IP packet is handed to the lower
   interface and before the ARP table is consulted.

5.4  IP Address

   The network manager assigns IP addresses to computers according to
   the IP network to which the computer is attached.  One part of a 4-
   byte IP address is the IP network number, the other part is the IP
   computer number (or host number).  For the computer in table 1, with
   an IP address of 223.1.2.1, the network number is 223.1.2 and the
   host number is number 1.

   The portion of the address that is used for network number and for
   host number is defined by the upper bits in the 4-byte address.  All
   example IP addresses in this tutorial are of type class C, meaning
   that the upper 3 bits indicate that 21 bits are the network number
   and 8 bits are the host number.  This allows 2,097,152 class C
   networks up to 254 hosts on each network.

   The IP address space is administered by the NIC (Network Information
   Center).  All internets that are connected to the single world-wide
   Internet must use network numbers assigned by the NIC.  If you are
   setting up your own internet and you are not intending to connect it
   to the Internet, you should still obtain your network numbers from
   the NIC.  If you pick your own number, you run the risk of confusion
   and chaos in the eventuality that your internet is connected to
   another internet.

5.5  Names

   People refer to computers by names, not numbers.  A computer called
   alpha might have the IP address of 223.1.2.1.  For small networks,
   this name-to-address translation data is often kept on each computer
   in the "hosts" file.  For larger networks, this translation data file
   is stored on a server and accessed across the network when needed.  A
   few lines from that file might look like this:

   223.1.2.1     alpha
   223.1.2.2     beta
   223.1.2.3     gamma
   223.1.2.4     delta
   223.1.3.2     epsilon
   223.1.4.2     iota



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   The IP address is the first column and the computer name is the
   second column.

   In most cases, you can install identical "hosts" files on all
   computers.  You may notice that "delta" has only one entry in this
   file even though it has 3 IP addresses.  Delta can be reached with
   any of its IP addresses; it does not matter which one is used.  When
   delta receives an IP packet and looks at the destination address, it
   will recognize any of its own IP addresses.

   IP networks are also given names.  If you have 3 IP networks, your
   "networks" file for documenting these names might look something like
   this:

   223.1.2     development
   223.1.3     accounting
   223.1.4     factory

   The IP network number is in the first column and its name is in the
   second column.

   From this example you can see that alpha is computer number 1 on the
   development network, beta is computer number 2 on the development
   network and so on.  You might also say that alpha is development.1,
   Beta is development.2, and so on.

   The above hosts file is adequate for the users, but the network
   manager will probably replace the line for delta with:

   223.1.2.4     devnetrouter    delta
   223.1.3.1     facnetrouter
   223.1.4.1     accnetrouter

   These three new lines for the hosts file give each of delta's IP
   addresses a meaningful name.  In fact, the first IP address listed
   has 2 names; "delta" and "devnetrouter" are synonyms.  In practice
   "delta" is the general-purpose name of the computer and the other 3
   names are only used when administering the IP route table.

   These files are used by network administration commands and network
   applications to provide meaningful names.  They are not required for
   operation of an internet, but they do make it easier for us.