组织:中国互动出版网(http://www.china-pub.com/)
RFC文档中文翻译计划(http://www.china-pub.com/compters/emook/aboutemook.htm)
E-mail:ouyang@china-pub.com
译者:吕新阳(lv_xinyang   xylv@travelsky.com)
译文发布时间:2002-4-23
版权:本中文翻译文档版权归中国互动出版网所有。可以用于非商业用途自由转载,但必须保留本文档的翻译及版权信息。

Network Working Group                                                 T. Pummill
Request for Comments: 1878                                              Alantec
Obsoletes: 1860                                                      B. Manning
Category: Informational                                                     ISI
            December 1995

IPv4可变长子网表
( RFC 1878 ——Variable Length Subnet Table For IPv4 )

本备忘录的状态
本文档讲述了一种Internet社区的信息,它不特指任何一种Internet标准。本备忘录的发布不受任何限制。
摘要
本文档通过描述一种标准子网表以统一关于IP网络子网问题的讨论.此表包括了A,B,C类网络的子网化分,以及网络表示,主机范围和C类子网的IP广播地址.
本文档可以作为关于子网划分RFC[1]和主机要求RFC[2]的参考.
介绍
自从STD 5,RFC 950和STD 3,RFC 1123提出后,网络的增长使得网络子网化分更加庞大复杂.上述RFCs都遵从子网创建的现存标准,但是有时也会出现因概念上的误解所导致的混淆.
本文对IPv4子网的所有可能值都做了分类.另外还对十六进制和十进制值,完全类等价法及在指示块中的地址数目作了附加说明.

下表列出了从1到32的所有可变长的子网,CIDR[3]表示法及十进制表示法.
(M = Million, K=Thousand, A,B,C= traditional class values)
  Mask value:                                # of
   Hex            CIDR   Decimal           addresses  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
   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        1/2 C
   FF.FF.FF.C0     /26     255.255.255.192     64        1/4 C
   FF.FF.FF.E0     /27     255.255.255.224     32        1/8 C
   FF.FF.FF.F0     /28     255.255.255.240     16       1/16 C
   FF.FF.FF.F8     /29     255.255.255.248      8       1/32 C
   FF.FF.FF.FC     /30     255.255.255.252      4       1/64 C
   FF.FF.FF.FE     /31     255.255.255.254      2      1/128 C
   FF.FF.FF.FF     /32     255.255.255.255   This is a single host route
子网与网络
可用的网络及主机地址的数目取决于用于掩码的位数.下表列出了对应于不同子网位数的网络,广播地址及主机地址.请注意:所有的全零和全一子网也被包括在表1-1和1-2中,这是依据现存的基于标准的为使用所有可定义的子网[4]应用而定的.
表1-1    传统的B类网络地址的子网划分
子网掩码      网络数      网络地址   主机地址范围      广播地址
位数          主机数

255.255.128.0   2 nets        N.N.0.0     N.N.0-127.N      N.N.127.255
1 bit subnet    32766         N.N.128.0   N.N.128-254.N    N.N.254.255

255.255.192.0   4 nets        N.N.0.0     N.N.0-63.N       N.N.63.255
2 bit subnet    16382         N.N.64.0    N.N.64-127.N     N.N.127.255
                           N.N.128.0   N.N.128-191.N    N.N.191.255
                           N.N.192.0   N.N.192-254.N    N.N.254.255

255.255.224.0   8 nets        N.N.0.0     N.N.0-31.N       N.N.31.255
3 bit subnet    8190          N.N.32.0    N.N.32-63.N      N.N.63.255
                           N.N.64.0    N.N.64-95.N      N.N.95.255
                            N.N.96.0    N.N.96-127.N     N.N.127.255
                            N.N.128.0   N.N.128-159.N    N.N.159.255
                            N.N.160.0   N.N.160-191.N    N.N.191.255
                            N.N.192.0   N.N.192-223.N    N.N.223.255
                            N.N.224.0   N.N.224-254.N    N.N.254.255

255.255.240.0   16 nets       N.N.0.0     N.N.0-15.N       N.N.15.255
4 bit subnet    4094          N.N.16.0    N.N.16-31.N      N.N.31.255
                           N.N.32.0    N.N.32-47.N      N.N.47.255
                           N.N.48.0    N.N.48-63.N      N.N.63.255
                           N.N.64.0    N.N.64-79.N      N.N.79.255
                           N.N.80.0    N.N.80-95.N      N.N.95.255
                           N.N.96.0    N.N.96-111.N     N.N.111.255
                           N.N.112.0   N.N.112-127.N    N.N.127.255
                           N.N.128.0   N.N.128-143.N    N.N.143.255
                           N.N.144.0   N.N.144-159.N    N.N.159.255
                           N.N.160.0   N.N.160-175.N    N.N.175.255
                           N.N.176.0   N.N.176-191.N    N.N.191.255
                           N.N.192.0   N.N.192-207.N    N.N.207.255
                           N.N.208.0   N.N.208-223.N    N.N.223.255
                           N.N.224.0   N.N.224-239.N    N.N.239.255
                           N.N.240.0   N.N.240-254.N    N.N.254.255
255.255.248.0   32 nets       N.N.0.0     N.N.0-7.N        N.N.7.255
5 bit subnet    2046          N.N.8.0     N.N.8-15.N       N.N.15.255
                           N.N.16.0    N.N.16-23.N      N.N.23.255
                           N.N.24.0    N.N.24-31.N      N.N.31.255
                           N.N.32.0    N.N.32-39.N      N.N.39.255
                           N.N.40.0    N.N.40-47.N      N.N.47.255
                           N.N.48.0    N.N.48-55.N      N.N.55.255
                           N.N.56.0    N.N.56-63.N      N.N.63.255
                           N.N.64.0    N.N.64-71.N      N.N.71.255
                           N.N.72.0    N.N.72-79.N      N.N.79.255
                           N.N.80.0    N.N.80-87.N      N.N.87.255
                        N.N.88.0    N.N.88-95.N      N.N.95.255
                            N.N.96.0    N.N.96-103.N     N.N.103.255
                            N.N.104.0   N.N.104-111.N    N.N.111.255
                            N.N.112.0   N.N.112-119.N    N.N.119.255
                            N.N.120.0   N.N.120-127.N    N.N.127.255
                            N.N.128.0   N.N.128-135.N    N.N.135.255
                            N.N.136.0   N.N.136-143.N    N.N.143.255
                            N.N.144.0   N.N.144-151.N    N.N.151.255
                            N.N.152.0   N.N.152-159.N    N.N.159.255
                            N.N.160.0   N.N.160-167.N    N.N.167.255
                            N.N.168.0   N.N.168-175.N    N.N.175.255
                            N.N.176.0   N.N.176-183.N    N.N.183.255
                            N.N.184.0   N.N.184-191.N    N.N.191.255
                            N.N.192.0   N.N.192-199.N    N.N.199.255
                            N.N.200.0   N.N.200-207.N    N.N.207.255
                            N.N.208.0   N.N.208-215.N    N.N.215.255
                            N.N.216.0   N.N.216-223.N    N.N.223.255
                            N.N.224.0   N.N.224-231.N    N.N.231.255
                            N.N.232.0   N.N.232-239.N    N.N.239.255
                            N.N.240.0   N.N.240-247.N    N.N.247.255
                            N.N.248.0   N.N.248-254.N    N.N.254.255

255.255.252.0   64 nets       N.N.0.0     N.N.0-3.N        N.N.3.255
6 bit subnet    1022          N.N.4.0     N.N.4-7.N        N.N.7.255
                           N.N.8.0     N.N.8-11.N       N.N.11.255
                           N.N.12.0    N.N.12-15.N      N.N.15.255
                           N.N.240.0   N.N.240-243.N    N.N.243.255
                           N.N.244.0   N.N.244-247.N    N.N.247.255
                           N.N.248.0   N.N.248-251.N    N.N.251.255
                           N.N.252.0   N.N.252-254.N    N.N.254.255

255.255.254.0   128 nets      N.N.0.0     N.N.0-1.N        N.N.1.255
7 bit subnet    510           N.N.2.0     N.N.2-3.N        N.N.3.255
                           N.N.4.0     N.N.4-5.N        N.N.5.255
                           N.N.250.0   N.N.250-251.N    N.N.251.255
                           N.N.252.0   N.N.252-253.N    N.N.253.255
                           N.N.254.0   N.N.254.N        N.N.254.255

255.255.255.0   255 nets      N.N.0.0     N.N.0.N          N.N.0.255
8 bit subnet    253           N.N.1.0     N.N.1.N          N.N.1.255
                           N.N.252.0   N.N.252.N        N.N.252.255
                           N.N.253.0   N.N.253.N        N.N.253.255
                           N.N.254.0   N.N.254.N        N.N.254.255

表1-2  传统C类网络地址的子网划分(类似于扩展的B类子网)

子网掩码      网络数      网络地址   主机地址范围      广播地址
位数          主机数
255.255.255.128 2 nets       N.N.N.0     N.N.N.1-126      N.N.N.127
1 bit Class C   126         N.N.N.128   N.N.N.129-254    N.N.N.255
9 bit Class B

255.255.255.192 4 nets       N.N.N.0     N.N.N.1-62       N.N.N.63
2 bit Class C   62          N.N.N.64    N.N.N.65-126     N.N.N.127
10 bit Class B              N.N.N.128   N.N.N.129-190    N.N.N.191
                         N.N.N.192   N.N.N.193-254    N.N.N.255

255.255.255.224 8 nets       N.N.N.0     N.N.N.1-30       N.N.N.31
3 bit Class C   30          N.N.N.32    N.N.N.33-62      N.N.N.63
11 bit Class B              N.N.N.64    N.N.N.65-94      N.N.N.95
                         N.N.N.96    N.N.N.97-126     N.N.N.127
                         N.N.N.128   N.N.N.129-158    N.N.N.159
                         N.N.N.160   N.N.N.161-190    N.N.N.191
                         N.N.N.192   N.N.N.193-222    N.N.N.223
                         N.N.N.224   N.N.N.225-254    N.N.N.255

255.255.255.240 16 nets     N.N.N.0     N.N.N.1-14       N.N.N.15
4 bit Class C   14          N.N.N.16    N.N.N.17-30      N.N.N.31
12 bit Class B              N.N.N.32    N.N.N.33-46      N.N.N.47
                         N.N.N.48    N.N.N.49-62      N.N.N.63
                         N.N.N.64    N.N.N.65-78      N.N.N.79
                         N.N.N.80    N.N.N.81-94      N.N.N.95
                         N.N.N.96    N.N.N.97-110     N.N.N.111
                         N.N.N.112   N.N.N.113-126    N.N.N.127
                         N.N.N.128   N.N.N.129-142    N.N.N.143
                         N.N.N.144   N.N.N.145-158    N.N.N.159
                         N.N.N.160   N.N.N.161-174    N.N.N.175
                         N.N.N.176   N.N.N.177-190    N.N.N.191
                         N.N.N.192   N.N.N.193-206    N.N.N.207

                         N.N.N.208   N.N.N.209-222    N.N.N.223
                         N.N.N.224   N.N.N.225-238    N.N.N.239
                         N.N.N.240   N.N.N.241-254    N.N.N.255

255.255.255.248 32 nets      N.N.N.0     N.N.N.1-6        N.N.N.7
5 bit Class C   6            N.N.N.8     N.N.N.9-14       N.N.N.15
13 bit Class B               N.N.N.16    N.N.N.17-22      N.N.N.23
                          N.N.N.24    N.N.N.25-30      N.N.N.31
                          N.N.N.32    N.N.N.33-38      N.N.N.39
                          N.N.N.40    N.N.N.41-46      N.N.N.47
                          N.N.N.48    N.N.N.49-54      N.N.N.55
                          N.N.N.56    N.N.N.57-62      N.N.N.63
                          N.N.N.64    N.N.N.65-70      N.N.N.71
                          N.N.N.72    N.N.N.73-78      N.N.N.79
                          N.N.N.80    N.N.N.81-86      N.N.N.87
                          N.N.N.88    N.N.N.89-94      N.N.N.95
                          N.N.N.96    N.N.N.97-102     N.N.N.103
                          N.N.N.104   N.N.N.105-110    N.N.N.111
                          N.N.N.112   N.N.N.113-118    N.N.N.119
                          N.N.N.120   N.N.N.121-126    N.N.N.127
                          N.N.N.128   N.N.N.129-134    N.N.N.135
                          N.N.N.136   N.N.N.137-142    N.N.N.143
                          N.N.N.144   N.N.N.145-150    N.N.N.151
                          N.N.N.152   N.N.N.153-158    N.N.N.159
                          N.N.N.160   N.N.N.161-166    N.N.N.167
                          N.N.N.168   N.N.N.169-174    N.N.N.175
                          N.N.N.176   N.N.N.177-182    N.N.N.183
                          N.N.N.184   N.N.N.185-190    N.N.N.191
                          N.N.N.192   N.N.N.193-198    N.N.N.199
                          N.N.N.200   N.N.N.201-206    N.N.N.207
                          N.N.N.208   N.N.N.209-214    N.N.N.215
                          N.N.N.216   N.N.N.217-222    N.N.N.223
                          N.N.N.224   N.N.N.225-230    N.N.N.231
                          N.N.N.232   N.N.N.233-238    N.N.N.239
                          N.N.N.240   N.N.N.241-246    N.N.N.247
                          N.N.N.248   N.N.N.249-254    N.N.N.255

255.255.255.252 64 nets       N.N.N.0     N.N.N.1-2        N.N.N.3
6 bit Class C   2             N.N.N.4     N.N.N.5-6        N.N.N.7
14 bit Class B                N.N.N.8     N.N.N.9-10       N.N.N.11
                           N.N.N.244   N.N.N.245-246    N.N.N.247
                           N.N.N.248   N.N.N.249-250    N.N.N.251
                           N.N.N.252   N.N.N.253-254    N.N.N.255
在本文档中,基于完备性的考虑,表2-1和2-2指出一些用于子网/主机部分的选项,以保证在制定的块中使用排除全零和全一子网[2]的算法.许多供应商只支持此类子网.注意:这种方法已经过时!新软件将使用所有可定义的网络.
表2-1  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

表 2-2   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

* 全零全一子网已被排除.  (过时的)
* 全零全一主机已被排除.  (过时的)
参考
[1]Mogul J., "BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS", STD 5, RFC 922, Stanford University, October 1984。
[2]Braden R., Editor, "Requirements for Internet Hosts -- Application and Support", STD 3, RFC 1123, IETF, October 1989。
[3]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。
[4]Baker F., "Requirements for IP Version 4 Routers", RFC 1812, cisco Systems, June 1995。
[5]Mogul J., and J. Postel, "Internet Standard Subnetting Procedure", STD 5, RFC 950, Stanford, USC/Information Sciences Institute, August 1985.
安全考虑
本文档不讨论安全问题。
作者地址:
Troy T. Pummill
Alantec
2115 O'Nel Drive
San Jose, CA  95131
USA
Phone: +1 408.467.4871
Fax:   +1 408.441.0272
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

RFC1878——Variable Length Subnet Table For IPv4                           IPv4可变长子网表




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