Network Working Group T. Pummill
Request for Comments: 1878 Alantec
Obsoletes: 1860 B. Manning
Category: Informational ISI
December 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 memo clarifies issues surrounding subnetting IP networks by
providing a standard subnet table. This table includes subnetting
for Class A, B, and C networks, as well as Network IDs, host ranges
and IP broadcast addresses with emphasis on Class C subnets.
This memo is intended as an informational companion to Subneting RFC
[1] and the Hosts Requirements RFC [2].
Introduction
The growth of networking since the time of STD 5, RFC 950 and STD 3,
RFC 1123 has resulted in larger and more complex network subnetting.
The previously mentioned RFCs comprise the available guidelines for
creating subnetted networks, however they have occassionaly been
misinterpreted leading to confusion regarding proper subnetting.
This document itemizes the potential values for IPv4 subnets.
Additional information is provided for Hex and Decmial values,
classfull equivalants, and number of addresses available within the
indicated block.
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RFC 1878 Subnet Table December 1995
Table
The following table lists the variable length subnets from 1 to 32,
the CIDR [3] representation form (/xx) and the Decmial equivalents.
(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
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RFC 1878 Subnet Table December 1995
Subnets and Networks
The number of available network and host addresses are derived from
the number of bits used for subnet masking. The tables below depict
the number of subnetting bits and the resulting network, broadcast
address, and host addresses. Please note that all-zeros and all-ones
subnets are included in Tables 1-1 and 1-2 per the current,
standards- based practice for using all definable subnets [4].
Table 1-1 represents traditional subnetting of a Class B network
address.
Subnet Mask # of nets Net. Addr. Host Addr Range Brodcast Addr.
Bits of Subnet hosts/subnet
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
Table 1-2 represents traditional subnetting of a Class C network
address (which is identical to extended Class B subnets).
Subnet Mask # of nets Net. Addr. Host Addr Range Brodcast Addr.
Bits of Subnet hosts/subnet
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
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
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RFC 1878 Subnet Table December 1995
For the sake of completeness within this memo, tables 2-1 and 2-2
illistrate some options for subnet/host partions within selected
block sizes using calculations which exclude all-zeros and all-ones
subnets [2]. Many vendors only support subnetting based upon this
premise. This practice is obsolete! Modern software will be able to
utilize all definable networks.
Table 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
Table 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
*Subnet all zeroes and all ones excluded. (Obsolete)
*Host all zeroes and all ones excluded. (Obsolete)
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RFC 1878 Subnet Table December 1995
References
[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.
Security Considerations
Security issues are not discussed in this memo.
Authors' Addresses
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
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