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Network Working Group                                         T. Pummill
Request for Comments: 1860                                       Alantec
Category: Informational                                       B. Manning
                                                                     ISI
                                                            October 1995


                 Variable Length Subnet Table For IPv4

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document itemizes the potential values for IPv4 subnets.
   Additional information is provided for Hex and Decmial values,
   classfull equivalents, and number of addresses available within the
   indicated block. We appreciate inputs from Bruce Pinsky (cisco) and
   Daniel Karrenberg (RIPE).

Table

   The following table lists the variable length subnets from 1 to 32,
   the CIDR representation form and the Decmial equivalents.

   Mask value:
   Hex            CIDR   Decimal           #of add    Classfull
   80.00.00.00    /1     128.0.0.0         2048 M     128 A
   C0.00.00.00    /2     192.0.0.0         1024 M      64 A
   E0.00.00.00    /3     224.0.0.0          512 M      32 A
   F0.00.00.00    /4     240.0.0.0          256 M      16 A
   F8.00.00.00    /5     248.0.0.0          128 M       8 A
   FC.00.00.00    /6     252.0.0.0           64 M       4 A
   FE.00.00.00    /7     254.0.0.0           32 M       2 A
   FF.00.00.00    /8     255.0.0.0           16 M       1 A
   FF.80.00.00    /9     255.128.0.0          8 M     128 B
   FF.C0.00.00   /10     255.192.0.0          4 M      64 B
   FF.E0.00.00   /11     255.224.0.0          2 M      32 B
   FF.F0.00.00   /12     255.240.0.0       1024 K      16 B
   FF.F8.00.00   /13     255.248.0.0        512 K       8 B
   FF.FC.00.00   /14     255.252.0.0        256 K       4 B
   FF.FE.00.00   /15     255.254.0.0        128 K       2 B
   FF.FF.00.00   /16     255.255.0.0         64 K       1 B
   FF.FF.80.00   /17     255.255.128.0       32 K     128 C
   FF.FF.C0.00   /18     255.255.192.0       16 K      64 C



Pummill & Manning            Informational                      [Page 1]

RFC 1860                      Subnet Table                  October 1995


   FF.FF.E0.00   /19     255.255.224.0        8 K      32 C
   FF.FF.F0.00   /20     255.255.240.0        4 K      16 C
   FF.FF.F8.00   /21     255.255.248.0        2 K       8 C
   FF.FF.FC.00   /22     255.255.252.0        1 K       4 C
   FF.FF.FE.00   /23     255.255.254.0      512         2 C
   FF.FF.FF.00   /24     255.255.255.0      256         1 C
   FF.FF.FF.80   /25     255.255.255.128    128
   FF.FF.FF.C0   /26     255.255.255.192     64
   FF.FF.FF.E0   /27     255.255.255.224     32
   FF.FF.FF.F0   /28     255.255.255.240     16
   FF.FF.FF.F8   /29     255.255.255.248      8
   FF.FF.FF.FC   /30     255.255.255.252      4
   FF.FF.FF.FE   /31     255.255.255.254   This space is not usable
   FF.FF.FF.FF   /32     255.255.255.255   This is a single host route

Examples

   The following tables illistrate some options for subnet/host partions
   within selected block sizes.

From a /16 block

   # bits          Mask            Effective Subnets     Effective Hosts
   ========        =====           =================     ===============
   2               255.255.192.0   2                       16382
   3               255.255.224.0   6                       8190
   4               255.255.240.0   14                      4094
   5               255.255.248.0   30                      2046
   6               255.255.252.0   62                      1022
   7               255.255.254.0   126                     510
   8               255.255.255.0   254                     254
   9               255.255.255.128 510                     126
   10              255.255.255.192 1022                    62
   11              255.255.255.224 2046                    30
   12              255.255.255.240 4094                    14
   13              255.255.255.248 8190                    6
   14              255.255.255.252 16382                   2














Pummill & Manning            Informational                      [Page 2]

RFC 1860                      Subnet Table                  October 1995


From a /24 block

   # bits          Mask            Effective Subnets     Effective Hosts
   ========        =====           =================     ===============
   2               255.255.255.192 2                       62
   3               255.255.255.224 6                       30
   4               255.255.255.240 14                      14
   5               255.255.255.248 30                      6
   6               255.255.255.252 62                      2

   *Subnet all zeroes and all ones excluded.
   *Host all zeroes and all ones excluded.

References

   [1] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
       Domain Routing (CIDR): an Address Assignment and Aggregation
       Strategy", RFC 1519, BARRNet, cicso, Merit, OARnet, September
       1993.

Security Considerations

   Security issues are not discussed in this memo.

Authors' Addresses

   Troy Pummill

   EMail: trop@alantec.com


   Bill Manning
   Information Sciences Institute
   University of Southern California
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695
   USA

   Phone: +1 310-822-1511 x387
   Fax:   +1 310-823-6714
   EMail: bmanning@isi.edu










Pummill & Manning            Informational                      [Page 3]