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  1.  
  2.  
  3.  
  4.  
  5.  
  6.  
  7. Network Working Group                                         T. Pummill
  8. Request for Comments: 1878                                       Alantec
  9. Obsoletes: 1860                                               B. Manning
  10. Category: Informational                                              ISI
  11.                                                            December 1995
  12.  
  13.  
  14.                  Variable Length Subnet Table For IPv4
  15.  
  16. Status of this Memo
  17.  
  18.    This memo provides information for the Internet community.  This memo
  19.    does not specify an Internet standard of any kind.  Distribution of
  20.    this memo is unlimited.
  21.  
  22. Abstract
  23.  
  24.    This memo clarifies issues surrounding subnetting IP networks by
  25.    providing a standard subnet table.  This table includes subnetting
  26.    for Class A, B, and C networks, as well as Network IDs, host ranges
  27.    and IP broadcast addresses with emphasis on Class C subnets.
  28.  
  29.    This memo is intended as an informational companion to Subneting RFC
  30.    [1] and the Hosts Requirements RFC [2].
  31.  
  32. Introduction
  33.  
  34.    The growth of networking since the time of STD 5, RFC 950 and STD 3,
  35.    RFC 1123 has resulted in larger and more complex network subnetting.
  36.    The previously mentioned RFCs comprise the available guidelines for
  37.    creating subnetted networks, however they have occassionaly been
  38.    misinterpreted leading to confusion regarding proper subnetting.
  39.  
  40.    This document itemizes the potential values for IPv4 subnets.
  41.    Additional information is provided for Hex and Decmial values,
  42.    classfull equivalants, and number of addresses available within the
  43.    indicated block.
  44.  
  45.  
  46.  
  47.  
  48.  
  49.  
  50.  
  51.  
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  54.  
  55.  
  56.  
  57.  
  58. Pummill & Manning            Informational                      [Page 1]
  59.  
  60. RFC 1878                      Subnet Table                 December 1995
  61.  
  62.  
  63. Table
  64.  
  65.    The following table lists the variable length subnets from 1 to 32,
  66.    the CIDR [3] representation form (/xx) and the Decmial equivalents.
  67.    (M = Million, K=Thousand, A,B,C= traditional class values)
  68.  
  69.    Mask value:                             # of
  70.    Hex            CIDR   Decimal           addresses  Classfull
  71.    80.00.00.00    /1     128.0.0.0         2048 M     128 A
  72.    C0.00.00.00    /2     192.0.0.0         1024 M      64 A
  73.    E0.00.00.00    /3     224.0.0.0          512 M      32 A
  74.    F0.00.00.00    /4     240.0.0.0          256 M      16 A
  75.    F8.00.00.00    /5     248.0.0.0          128 M       8 A
  76.    FC.00.00.00    /6     252.0.0.0           64 M       4 A
  77.    FE.00.00.00    /7     254.0.0.0           32 M       2 A
  78.    FF.00.00.00    /8     255.0.0.0           16 M       1 A
  79.    FF.80.00.00    /9     255.128.0.0          8 M     128 B
  80.    FF.C0.00.00   /10     255.192.0.0          4 M      64 B
  81.    FF.E0.00.00   /11     255.224.0.0          2 M      32 B
  82.    FF.F0.00.00   /12     255.240.0.0       1024 K      16 B
  83.    FF.F8.00.00   /13     255.248.0.0        512 K       8 B
  84.    FF.FC.00.00   /14     255.252.0.0        256 K       4 B
  85.    FF.FE.00.00   /15     255.254.0.0        128 K       2 B
  86.    FF.FF.00.00   /16     255.255.0.0         64 K       1 B
  87.    FF.FF.80.00   /17     255.255.128.0       32 K     128 C
  88.    FF.FF.C0.00   /18     255.255.192.0       16 K      64 C
  89.    FF.FF.E0.00   /19     255.255.224.0        8 K      32 C
  90.    FF.FF.F0.00   /20     255.255.240.0        4 K      16 C
  91.    FF.FF.F8.00   /21     255.255.248.0        2 K       8 C
  92.    FF.FF.FC.00   /22     255.255.252.0        1 K       4 C
  93.    FF.FF.FE.00   /23     255.255.254.0      512         2 C
  94.    FF.FF.FF.00   /24     255.255.255.0      256         1 C
  95.    FF.FF.FF.80   /25     255.255.255.128    128       1/2 C
  96.    FF.FF.FF.C0   /26     255.255.255.192     64       1/4 C
  97.    FF.FF.FF.E0   /27     255.255.255.224     32       1/8 C
  98.    FF.FF.FF.F0   /28     255.255.255.240     16      1/16 C
  99.    FF.FF.FF.F8   /29     255.255.255.248      8      1/32 C
  100.    FF.FF.FF.FC   /30     255.255.255.252      4      1/64 C
  101.    FF.FF.FF.FE   /31     255.255.255.254      2     1/128 C
  102.    FF.FF.FF.FF   /32     255.255.255.255   This is a single host route
  103.  
  104.  
  105.  
  106.  
  107.  
  108.  
  109.  
  110.  
  111.  
  112.  
  113.  
  114. Pummill & Manning            Informational                      [Page 2]
  115.  
  116. RFC 1878                      Subnet Table                 December 1995
  117.  
  118.  
  119. Subnets and Networks
  120.  
  121.    The number of available network and host addresses are derived from
  122.    the number of bits used for subnet masking.  The tables below depict
  123.    the number of subnetting bits and the resulting network, broadcast
  124.    address, and host addresses.  Please note that all-zeros and all-ones
  125.    subnets are included in Tables 1-1 and 1-2 per the current,
  126.    standards- based practice for using all definable subnets [4].
  127.  
  128.    Table 1-1 represents traditional subnetting of a Class B network
  129.    address.
  130.  
  131. Subnet Mask     # of nets    Net. Addr.  Host Addr Range  Brodcast Addr.
  132. Bits of Subnet  hosts/subnet
  133.  
  134. 255.255.128.0   2 nets        N.N.0.0     N.N.0-127.N      N.N.127.255
  135. 1 bit subnet    32766         N.N.128.0   N.N.128-254.N    N.N.254.255
  136.  
  137. 255.255.192.0   4 nets        N.N.0.0     N.N.0-63.N       N.N.63.255
  138. 2 bit subnet    16382         N.N.64.0    N.N.64-127.N     N.N.127.255
  139.                               N.N.128.0   N.N.128-191.N    N.N.191.255
  140.                               N.N.192.0   N.N.192-254.N    N.N.254.255
  141.  
  142. 255.255.224.0   8 nets        N.N.0.0     N.N.0-31.N       N.N.31.255
  143. 3 bit subnet    8190          N.N.32.0    N.N.32-63.N      N.N.63.255
  144.                               N.N.64.0    N.N.64-95.N      N.N.95.255
  145.                               N.N.96.0    N.N.96-127.N     N.N.127.255
  146.                               N.N.128.0   N.N.128-159.N    N.N.159.255
  147.                               N.N.160.0   N.N.160-191.N    N.N.191.255
  148.                               N.N.192.0   N.N.192-223.N    N.N.223.255
  149.                               N.N.224.0   N.N.224-254.N    N.N.254.255
  150.  
  151. 255.255.240.0   16 nets       N.N.0.0     N.N.0-15.N       N.N.15.255
  152. 4 bit subnet    4094          N.N.16.0    N.N.16-31.N      N.N.31.255
  153.                               N.N.32.0    N.N.32-47.N      N.N.47.255
  154.                               N.N.48.0    N.N.48-63.N      N.N.63.255
  155.                               N.N.64.0    N.N.64-79.N      N.N.79.255
  156.                               N.N.80.0    N.N.80-95.N      N.N.95.255
  157.                               N.N.96.0    N.N.96-111.N     N.N.111.255
  158.                               N.N.112.0   N.N.112-127.N    N.N.127.255
  159.                               N.N.128.0   N.N.128-143.N    N.N.143.255
  160.                               N.N.144.0   N.N.144-159.N    N.N.159.255
  161.                               N.N.160.0   N.N.160-175.N    N.N.175.255
  162.                               N.N.176.0   N.N.176-191.N    N.N.191.255
  163.                               N.N.192.0   N.N.192-207.N    N.N.207.255
  164.                               N.N.208.0   N.N.208-223.N    N.N.223.255
  165.                               N.N.224.0   N.N.224-239.N    N.N.239.255
  166.                               N.N.240.0   N.N.240-254.N    N.N.254.255
  167.  
  168.  
  169.  
  170. Pummill & Manning            Informational                      [Page 3]
  171.  
  172. RFC 1878                      Subnet Table                 December 1995
  173.  
  174.  
  175. 255.255.248.0   32 nets       N.N.0.0     N.N.0-7.N        N.N.7.255
  176. 5 bit subnet    2046          N.N.8.0     N.N.8-15.N       N.N.15.255
  177.                               N.N.16.0    N.N.16-23.N      N.N.23.255
  178.                               N.N.24.0    N.N.24-31.N      N.N.31.255
  179.                               N.N.32.0    N.N.32-39.N      N.N.39.255
  180.                               N.N.40.0    N.N.40-47.N      N.N.47.255
  181.                               N.N.48.0    N.N.48-55.N      N.N.55.255
  182.                               N.N.56.0    N.N.56-63.N      N.N.63.255
  183.                               N.N.64.0    N.N.64-71.N      N.N.71.255
  184.                               N.N.72.0    N.N.72-79.N      N.N.79.255
  185.                               N.N.80.0    N.N.80-87.N      N.N.87.255
  186.                               N.N.88.0    N.N.88-95.N      N.N.95.255
  187.                               N.N.96.0    N.N.96-103.N     N.N.103.255
  188.                               N.N.104.0   N.N.104-111.N    N.N.111.255
  189.                               N.N.112.0   N.N.112-119.N    N.N.119.255
  190.                               N.N.120.0   N.N.120-127.N    N.N.127.255
  191.                               N.N.128.0   N.N.128-135.N    N.N.135.255
  192.                               N.N.136.0   N.N.136-143.N    N.N.143.255
  193.                               N.N.144.0   N.N.144-151.N    N.N.151.255
  194.                               N.N.152.0   N.N.152-159.N    N.N.159.255
  195.                               N.N.160.0   N.N.160-167.N    N.N.167.255
  196.                               N.N.168.0   N.N.168-175.N    N.N.175.255
  197.                               N.N.176.0   N.N.176-183.N    N.N.183.255
  198.                               N.N.184.0   N.N.184-191.N    N.N.191.255
  199.                               N.N.192.0   N.N.192-199.N    N.N.199.255
  200.                               N.N.200.0   N.N.200-207.N    N.N.207.255
  201.                               N.N.208.0   N.N.208-215.N    N.N.215.255
  202.                               N.N.216.0   N.N.216-223.N    N.N.223.255
  203.                               N.N.224.0   N.N.224-231.N    N.N.231.255
  204.                               N.N.232.0   N.N.232-239.N    N.N.239.255
  205.                               N.N.240.0   N.N.240-247.N    N.N.247.255
  206.                               N.N.248.0   N.N.248-254.N    N.N.254.255
  207.  
  208. 255.255.252.0   64 nets       N.N.0.0     N.N.0-3.N        N.N.3.255
  209. 6 bit subnet    1022          N.N.4.0     N.N.4-7.N        N.N.7.255
  210.                               N.N.8.0     N.N.8-11.N       N.N.11.255
  211.                               N.N.12.0    N.N.12-15.N      N.N.15.255
  212.                               N.N.240.0   N.N.240-243.N    N.N.243.255
  213.                               N.N.244.0   N.N.244-247.N    N.N.247.255
  214.                               N.N.248.0   N.N.248-251.N    N.N.251.255
  215.                               N.N.252.0   N.N.252-254.N    N.N.254.255
  216.  
  217.  
  218. 255.255.254.0   128 nets      N.N.0.0     N.N.0-1.N        N.N.1.255
  219. 7 bit subnet    510           N.N.2.0     N.N.2-3.N        N.N.3.255
  220.                               N.N.4.0     N.N.4-5.N        N.N.5.255
  221.                               N.N.250.0   N.N.250-251.N    N.N.251.255
  222.                               N.N.252.0   N.N.252-253.N    N.N.253.255
  223.  
  224.  
  225.  
  226. Pummill & Manning            Informational                      [Page 4]
  227.  
  228. RFC 1878                      Subnet Table                 December 1995
  229.  
  230.  
  231.                               N.N.254.0   N.N.254.N        N.N.254.255
  232.  
  233.  
  234. 255.255.255.0   255 nets      N.N.0.0     N.N.0.N          N.N.0.255
  235. 8 bit subnet    253           N.N.1.0     N.N.1.N          N.N.1.255
  236.                               N.N.252.0   N.N.252.N        N.N.252.255
  237.                               N.N.253.0   N.N.253.N        N.N.253.255
  238.                               N.N.254.0   N.N.254.N        N.N.254.255
  239.  
  240.    Table 1-2 represents traditional subnetting of a Class C network
  241.    address (which is identical to extended Class B subnets).
  242.  
  243. Subnet Mask     # of nets    Net. Addr.  Host Addr Range  Brodcast Addr.
  244. Bits of Subnet  hosts/subnet
  245.  
  246. 255.255.255.128 2 nets       N.N.N.0     N.N.N.1-126      N.N.N.127
  247. 1 bit Class C   126          N.N.N.128   N.N.N.129-254    N.N.N.255
  248. 9 bit Class B
  249.  
  250.  
  251.  
  252. 255.255.255.192 4 nets       N.N.N.0     N.N.N.1-62       N.N.N.63
  253. 2 bit Class C   62           N.N.N.64    N.N.N.65-126     N.N.N.127
  254. 10 bit Class B               N.N.N.128   N.N.N.129-190    N.N.N.191
  255.                              N.N.N.192   N.N.N.193-254    N.N.N.255
  256.  
  257. 255.255.255.224 8 nets       N.N.N.0     N.N.N.1-30       N.N.N.31
  258. 3 bit Class C   30           N.N.N.32    N.N.N.33-62      N.N.N.63
  259. 11 bit Class B               N.N.N.64    N.N.N.65-94      N.N.N.95
  260.                              N.N.N.96    N.N.N.97-126     N.N.N.127
  261.                              N.N.N.128   N.N.N.129-158    N.N.N.159
  262.                              N.N.N.160   N.N.N.161-190    N.N.N.191
  263.                              N.N.N.192   N.N.N.193-222    N.N.N.223
  264.                              N.N.N.224   N.N.N.225-254    N.N.N.255
  265.  
  266. 255.255.255.240 16 nets      N.N.N.0     N.N.N.1-14       N.N.N.15
  267. 4 bit Class C   14           N.N.N.16    N.N.N.17-30      N.N.N.31
  268. 12 bit Class B               N.N.N.32    N.N.N.33-46      N.N.N.47
  269.                              N.N.N.48    N.N.N.49-62      N.N.N.63
  270.                              N.N.N.64    N.N.N.65-78      N.N.N.79
  271.                              N.N.N.80    N.N.N.81-94      N.N.N.95
  272.                              N.N.N.96    N.N.N.97-110     N.N.N.111
  273.                              N.N.N.112   N.N.N.113-126    N.N.N.127
  274.                              N.N.N.128   N.N.N.129-142    N.N.N.143
  275.                              N.N.N.144   N.N.N.145-158    N.N.N.159
  276.                              N.N.N.160   N.N.N.161-174    N.N.N.175
  277.                              N.N.N.176   N.N.N.177-190    N.N.N.191
  278.                              N.N.N.192   N.N.N.193-206    N.N.N.207
  279.  
  280.  
  281.  
  282. Pummill & Manning            Informational                      [Page 5]
  283.  
  284. RFC 1878                      Subnet Table                 December 1995
  285.  
  286.  
  287.                              N.N.N.208   N.N.N.209-222    N.N.N.223
  288.                              N.N.N.224   N.N.N.225-238    N.N.N.239
  289.                              N.N.N.240   N.N.N.241-254    N.N.N.255
  290.  
  291.  
  292. 255.255.255.248 32 nets      N.N.N.0     N.N.N.1-6        N.N.N.7
  293. 5 bit Class C   6            N.N.N.8     N.N.N.9-14       N.N.N.15
  294. 13 bit Class B               N.N.N.16    N.N.N.17-22      N.N.N.23
  295.                              N.N.N.24    N.N.N.25-30      N.N.N.31
  296.                              N.N.N.32    N.N.N.33-38      N.N.N.39
  297.                              N.N.N.40    N.N.N.41-46      N.N.N.47
  298.                              N.N.N.48    N.N.N.49-54      N.N.N.55
  299.                              N.N.N.56    N.N.N.57-62      N.N.N.63
  300.                              N.N.N.64    N.N.N.65-70      N.N.N.71
  301.                              N.N.N.72    N.N.N.73-78      N.N.N.79
  302.                              N.N.N.80    N.N.N.81-86      N.N.N.87
  303.                              N.N.N.88    N.N.N.89-94      N.N.N.95
  304.                              N.N.N.96    N.N.N.97-102     N.N.N.103
  305.                              N.N.N.104   N.N.N.105-110    N.N.N.111
  306.                              N.N.N.112   N.N.N.113-118    N.N.N.119
  307.                              N.N.N.120   N.N.N.121-126    N.N.N.127
  308.                              N.N.N.128   N.N.N.129-134    N.N.N.135
  309.                              N.N.N.136   N.N.N.137-142    N.N.N.143
  310.                              N.N.N.144   N.N.N.145-150    N.N.N.151
  311.                              N.N.N.152   N.N.N.153-158    N.N.N.159
  312.                              N.N.N.160   N.N.N.161-166    N.N.N.167
  313.                              N.N.N.168   N.N.N.169-174    N.N.N.175
  314.                              N.N.N.176   N.N.N.177-182    N.N.N.183
  315.                              N.N.N.184   N.N.N.185-190    N.N.N.191
  316.                              N.N.N.192   N.N.N.193-198    N.N.N.199
  317.                              N.N.N.200   N.N.N.201-206    N.N.N.207
  318.                              N.N.N.208   N.N.N.209-214    N.N.N.215
  319.                              N.N.N.216   N.N.N.217-222    N.N.N.223
  320.                              N.N.N.224   N.N.N.225-230    N.N.N.231
  321.                              N.N.N.232   N.N.N.233-238    N.N.N.239
  322.                              N.N.N.240   N.N.N.241-246    N.N.N.247
  323.                              N.N.N.248   N.N.N.249-254    N.N.N.255
  324.  
  325. 255.255.255.252 64 nets      N.N.N.0     N.N.N.1-2        N.N.N.3
  326. 6 bit Class C   2            N.N.N.4     N.N.N.5-6        N.N.N.7
  327. 14 bit Class B               N.N.N.8     N.N.N.9-10       N.N.N.11
  328.                              N.N.N.244   N.N.N.245-246    N.N.N.247
  329.                              N.N.N.248   N.N.N.249-250    N.N.N.251
  330.                              N.N.N.252   N.N.N.253-254    N.N.N.255
  331.  
  332.  
  333.  
  334.  
  335.  
  336.  
  337.  
  338. Pummill & Manning            Informational                      [Page 6]
  339.  
  340. RFC 1878                      Subnet Table                 December 1995
  341.  
  342.  
  343.    For the sake of completeness within this memo, tables 2-1 and 2-2
  344.    illistrate some options for subnet/host partions within selected
  345.    block sizes using calculations which exclude all-zeros and all-ones
  346.    subnets [2].  Many vendors only support subnetting based upon this
  347.    premise.  This practice is obsolete!  Modern software will be able to
  348.    utilize all definable networks.
  349.  
  350. Table 2-1 from a /16 block
  351. # bits          Mask            Effective Subnets       Effective Hosts
  352. ========        =====           =================       ===============
  353. 2               255.255.192.0   2                       16382
  354. 3               255.255.224.0   6                       8190
  355. 4               255.255.240.0   14                      4094
  356. 5               255.255.248.0   30                      2046
  357. 6               255.255.252.0   62                      1022
  358. 7               255.255.254.0   126                     510
  359. 8               255.255.255.0   254                     254
  360. 9               255.255.255.128 510                     126
  361. 10              255.255.255.192 1022                    62
  362. 11              255.255.255.224 2046                    30
  363. 12              255.255.255.240 4094                    14
  364. 13              255.255.255.248 8190                    6
  365. 14              255.255.255.252 16382                   2
  366.  
  367. Table 2-2 from a /24 block
  368. # bits          Mask            Effective Subnets       Effective Hosts
  369. ========        =====           =================       ===============
  370. 2               255.255.255.192 2                       62
  371. 3               255.255.255.224 6                       30
  372. 4               255.255.255.240 14                      14
  373. 5               255.255.255.248 30                      6
  374. 6               255.255.255.252 62                      2
  375.  
  376. *Subnet all zeroes and all ones excluded. (Obsolete)
  377. *Host all zeroes and all ones excluded.   (Obsolete)
  378.  
  379.  
  380.  
  381.  
  382.  
  383.  
  384.  
  385.  
  386.  
  387.  
  388.  
  389.  
  390.  
  391.  
  392.  
  393.  
  394. Pummill & Manning            Informational                      [Page 7]
  395.  
  396. RFC 1878                      Subnet Table                 December 1995
  397.  
  398.  
  399. References
  400.  
  401.    [1] Mogul J., "BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF
  402.        SUBNETS", STD 5, RFC 922, Stanford University, October 1984.
  403.  
  404.    [2] Braden R., Editor, "Requirements for Internet Hosts --
  405.        Application and Support", STD 3, RFC 1123, IETF, October 1989.
  406.  
  407.    [3] Fuller V., Li T., Yu J., and K. Varadhan, "Classless Inter-
  408.        Domain Routing (CIDR): an Address Assignment and Aggregation
  409.        Strategy", RFC 1519, BARRNet, cicso, Merit, OARnet, September
  410.        1993.
  411.  
  412.    [4] Baker F., "Requirements for IP Version 4 Routers", RFC 1812,
  413.        cisco Systems, June 1995.
  414.  
  415.    [5] Mogul J., and J. Postel, "Internet Standard Subnetting
  416.        Procedure", STD 5, RFC 950, Stanford, USC/Information Sciences
  417.        Institute, August 1985.
  418.  
  419. Security Considerations
  420.  
  421.    Security issues are not discussed in this memo.
  422.  
  423. Authors' Addresses
  424.  
  425.    Troy T. Pummill
  426.    Alantec
  427.    2115 O'Nel Drive
  428.    San Jose, CA  95131
  429.    USA
  430.  
  431.    Phone: +1 408.467.4871
  432.    Fax:   +1 408.441.0272
  433.    EMail: trop@alantec.com
  434.  
  435.  
  436.    Bill Manning
  437.    Information Sciences Institute
  438.    University of Southern California
  439.    4676 Admiralty Way
  440.    Marina del Rey, CA 90292-6695
  441.    USA
  442.  
  443.    Phone: +1 310-822-1511 x387
  444.    Fax:   +1 310-823-6714
  445.    EMail: bmanning@isi.edu
  446.  
  447.  
  448.  
  449.  
  450. Pummill & Manning            Informational                      [Page 8]
  451.  
  452.