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Network Working Group                                          Z. WangRequest for Comments: 1335                                J. Crowcroft                                             University College London                                                              May 1992A Two-Tier Address Structure for the Internet:A Solution to the Problem of Address Space ExhaustionStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard.  Distribution of this memo is   unlimited.Abstract   This RFC presents a solution to problem of address space exhaustion   in the Internet.  It proposes a two-tier address structure for the   Internet.  This is an "idea" paper and discussion is strongly   encouraged.Introduction   Address space exhaustion is one of the most serious and immediate   problems that the Internet faces today [1,2].  The current Internet   address space is 32-bit.  Each Internet address is divided into two   parts: a network portion and a host portion.  This division   corresponds the three primary Internet address classes: Class A,   Class B and Class C.  Table 1 lists the network number statistics as   of April 1992.                      Total       Allocated     Allocated (%)   Class A              126            48            54%   Class B            16383          7006            43%   Class C          2097151         40724             2%          Table 1: Network Number Statistics (April 1992)   If recent trends of exponential growth continue, the network numbers   in Class B will soon run out [1,2].  There are over 2 million Class C   network numbers and only 2% have been allocated.  However, a Class C   network number can only accommodate 254 host numbers which is too   small for most networks.  With the rapid expansion of the Internet   and drastic increase in personal computers, the time when the 32-bit   address space is exhausted altogether is also not too distant [1-3].   Recently several proposals have been put forward to deal with theWang & Crowcroft                                                [Page 1]

RFC 1335      Two-Tier Address Structure for the Internet       May 1992   immediate problem [1-4].  The Supernetting and C-sharp schemes   attempt to make the Class C numbers more usable by re-defining the   way in which Class C network numbers are classified and assigned   [3,4].  Both schemes require modifications to the exterior routing   algorithms and global coordination across the Internet may be   required for the deployment.  The two schemes do not expand the total   number of addresses available to the Internet and therefore can only   be used as a short-term fix for next two or three years.  Schemes   have also been put forwarded in which the 32-bit address field is   replaced with a field of the same size but with different meaning and   the gateways on the boundary re-write the address when the packet   crossed the boundary [1,2,5].  Such schemes, however, requires   substantial changes to the gateways and the exterior routing   algorithm.   In this paper, we present an alternative solution to the problem of   address space exhaustion.  The "Dual Network Addressing (DNA)" scheme   proposed here is based on a two-tier address structure and sharing of   addresses.  It requires no modifications to the exterior routing   algorithms and any networks can adopt the scheme individually at any   time without affecting other networks.The Scheme   The DNA scheme attempts to reduce the waste in using the Internet   addresses.  A useful analogy to our scheme is the extension system   used in the telephone system.  Many large organizations usually have   extensive private telephone networks for internal use and at the mean   time hire a limited number of external lines for communications with   the outside world.  In such a telephone system, important offices may   have direct external lines and telephones in the public areas may be   restricted to internal calls only.  The majority of the telephones   can usually make both internal calls and external calls.  But they   must share a limited number of external lines.  When an external call   is being made, a pre-defined digit has to be pressed so that an   external line can be allocated from the poll of external lines.   In the DNA scheme, there are two types of Internet addresses:   Internal addresses and External addresses.  An internal address is an   Internet address only used within one network and is unique only   within that network.  An interface with an internal address can only   communicate with another interface with an internal address in the   same network.  An external address is unique in the entire Internet   and an interface with an external address can communicate directly to   another interface with an external address over the Internet.  All   current Internet addresses are external addresses.   In effect, the external addresses form one global Internet and theWang & Crowcroft                                                [Page 2]

RFC 1335      Two-Tier Address Structure for the Internet       May 1992   internal addresses form many private Internets.  Within one network,   the external addresses are only used for inter-network communications   and internal addresses for intra-network communications.  An External   Address Sharing Service (EASS) is needed to manage the sharing of   external addresses.  An EASS server reserves a number of external   addresses.  When a machine that only has an internal address wants to   communicate a machine with an external address in other networks, it   can send a request to an EASS server to obtain a temporary external   address.  After the use, the machine can return the external address   to the EASS server.   We believe that, with the DNA scheme, a network can operate with a   limited number of external addresses.  The reasons are as follows:   *  In most networks, the majority of the traffic is confined to      its local area networks.  This is due the nature of      networking applications and the bandwidth constraints on      inter-network links.   *  The number of machines which act as Internet servers, i.e.,      running programs waiting to be called by machines in other      networks, is often limited and certainly much smaller than      the total number of machines.  These machines include mail      servers, domain name servers, ftp archive servers, directory      servers, etc.   *  There are an increasingly large number of personal machines      entering the Internet.  The use of these machines is      primarily limited to their local environment.  They may also      be used as "clients" such as ftp and telnet to access other      machines.   *  For security reasons, many large organizations, such as banks,      government departments, military institution and some      companies, may only allow a very limited number of their      machines to have access to the global Internet.  The majority      of their machines are purely for internal use.   In the DNA scheme, all machines in a network are assigned a permanent   internal address and can communicate with any machines within the   same network.  The allocation of external addresses depends on the   functions of the machines and as a result it creates three-level   privileges:   *  machines which act as servers or used as central computing      infrastructure are likely to have frequent communications      with other networks therefore they may require external      addresses all the time.  These machines are allocatedWang & Crowcroft                                                [Page 3]

RFC 1335      Two-Tier Address Structure for the Internet       May 1992      permanent external addresses.   *  machines which are not allowed to communicate with other      networks have no external addresses and can only communicate      with machines within their own network.   *  the rest of the machines share a number of external      addresses. The external addresses are allocated by      the EASS server on request.  These machines can only      used as clients to call machines in other networks,      i.e., they can not be called by machines in other networks.   A network can choose any network number other than its external   network number as its internal network number.  Different networks   can use the same network number as their internal number.  We propose   to reserve one Class A network number as the well-known network   number for internal use.The Advantages   The DNA scheme attempts to tackle the problem from the bottom of the   Internet, i.e., each individual network, while other schemes   described in the first section deal with the problem from the top of   the Internet, i.e., gateways and exterior routing algorithms.  These   schemes, however, do not need to be consider as mutually exclusive.   The DNA scheme has several advantages:   *  The DNA scheme takes an evolutionary approach towards the      changes.  Different networks can individually choose to      adopt the scheme at any time only when necessary.      There is no need for global coordination between different      networks for their deployment.  The effects of the deployment      are confined to the network in which the scheme is being      implemented, and are invisible to exterior routing      algorithms and external networks.   *  With the DNA scheme, it is possible for a medium size organization      to use a Class C network number with 254 external addresses.      The scheme allows the current Internet to expand to over 2 million      networks and each network to have more than 16 million hosts.      This will allow considerable time for a long-term solution to      be developed and fully tested.   *  The DNA scheme requires modifications to the host software.      However, the modifications are needed only in those networks      which adopt the DNA scheme.   Since all existing Class A and B      networks usually have sufficient external addresses for all their      machines, they do not need to adopt the DNA scheme, and thereforeWang & Crowcroft                                                [Page 4]

RFC 1335      Two-Tier Address Structure for the Internet       May 1992      need no modifications at all to their software.  The networks      which need to use the DNA scheme are those new networks which are      set up after the Class A and B numbers run out and have to      use a Class C number.   *  The DNA scheme makes it possible to develop to a new addressing      scheme without expanding the 32-bit address length to 64-bit.      With the two-tier address structure, the current 32-bit space      can accommodate over 4 billion hosts in the global Internet and      100 million hosts in each individual network.  When we move to a      classless multi-hierarchic addressing scheme, the use of external      addresses can be more efficient and less wasteful and the      32-bit space can be adequate for the external addresses.   *  When a new addressing scheme has been developed, all current      Internet addresses have to be changed.  The DNA scheme will make      such a undertaking much easier and smoother, since only the      EASS servers and those have permanent external addresses will      be affected, and communications within the network will not      be interrupted.The Modifications   The major modifications to the host software is in the network   interface code.  The DNA scheme requires each machine to have at   least two addresses.  But most of the host software currently does   not allow us to bind two addresses to one physical interface.  This   problem can be solved by using two network interfaces on each   machine.  But this option is too expensive.  Note the two interfaces   are actually connected to the same physical network.  Therefore, if   we modify the interface code to allow two logical interfaces to be   mapped onto one single physical interface, the machine can then use   both the external address and the internal address with one physical   interface as if it has two physical interfaces.  In effect, two   logical IP networks operate over the same physical network.   The DNA scheme also has implications to the DNS service.  Many   machines will have two entries in the local name server.  The DNS   server must examine the source address of the request and decide   which entry to use.  If the source address matches the well-known   internal network number, it passes the internal address of the domain   name.  Otherwise, the name server passes the external address.   An EASS server is required to manage the sharing of the external   addresses, i.e., to allocate and de-allocate external addresses to   the machines which do not have permanent external addresses.  This   service can be provided by using the "Dynamic Host Configuration   Protocol (DHCP)" [6].Wang & Crowcroft                                                [Page 5]

RFC 1335      Two-Tier Address Structure for the Internet       May 1992   Many hosts do an inverse lookup of incoming connections.  Therefore,   it is desirable the entry in the DNS server be updated whenever a new   external address is allocated.  This will also allow an machine which   currently has a temporary external address to be called by other   machines.  The updating of the entry in the DNS server can be done   more easily if the EASS server and DNS server are co-located.Acknowledgements   We would like to thank J. K. Reynolds for the network statistics, and   V. Cerf, C. Topolcic, K. McCloghrie, R. Ullmann and K. Carlberg for   their useful comments and discussion.References   [1]  Chiappa, N., "The IP Addressing Issue", work in progress,        October 1990.   [2]  Clark, D., Chapin, L., Cerf, V., Braden, R., and R. Hobby,        "Towards the Future Architecture",RFC 1287, MIT, BBN, CNRI,        ISI, UC Davis, December 1991.   [3]  Solensky, F., and F. Kastenholz, "A Revision to IP Address        Classifications", work in progress, March 1992.   [4]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:        an Address Assignment and Aggregation Strategy", work in        progress, March 1992.   [5]  Tsuchiya, P., "The IP Network Address Translator", work in        progress, March 1991.   [6]  Droms, R., "Dynamic Host Configuration Protocol", work in        progress, March 1992.Wang & Crowcroft                                                [Page 6]

RFC 1335      Two-Tier Address Structure for the Internet       May 1992Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Zheng Wang   Dept. of Computer Science   University College London   London WC1E 6BT, UK   EMail: z.wang@cs.ucl.ac.uk   Jon Crowcroft   Dept. of Computer Science   University College London   London WC1E 6BT, UK   EMail: j.crowcroft@cs.ucl.ac.ukWang & Crowcroft                                                [Page 7]