RFC1860 - Variable Length Subnet Table For IPv4

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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

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

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", RFC1519, 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

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