IPv4 VLSM Table: Variable Length Subnet Table For IPv4
A reference table of IPv4 subnet masks and the address blocks they describe.
Pummill, T. and B. Manning, "Variable Length Subnet Table For IPv4", RFC 1860, DOI 10.17487/RFC1860, October 1995, <https://www.rfc-editor.org/info/rfc1860>.
@misc{rfc1860,
series = {Request for Comments},
number = 1860,
howpublished = {RFC 1860},
publisher = {RFC Editor},
doi = {10.17487/RFC1860},
url = {https://www.rfc-editor.org/info/rfc1860},
author = {T. Pummill and B. Manning},
title = {{Variable Length Subnet Table For IPv4}},
pagetotal = 3,
year = 1995,
month = oct,
} RFC text
Contents
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", 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
ISO Plain Language
On this page
§1What the Table Contains
The memo itemizes potential values for IPv4 subnets. For each listed mask it gives hexadecimal and decimal forms, CIDR notation, a classful equivalent where shown, and the number of addresses in the block.
The mask list runs from /1 to /32. The memo is a reference table; it does not define a protocol for allocating or routing those blocks.
The table lets readers look up an IPv4 mask in different notations. It also shows the size of the address block represented by each mask.
§2Mask Notations and Address Counts
Each table row pairs a mask's hexadecimal value with CIDR notation and a dotted-decimal value. The table also gives an approximate number of addresses using K and M units, and lists a Class A, B, or C equivalent for some masks. For example, the row for /24 lists 255.255.255.0 and 256 addresses.
The last rows mark /31 as unusable and describe /32 as a single host route. The RFC does not provide a classful equivalent for every row.
Find a mask's row to see its CIDR and dotted-decimal forms and the block size. The table also marks /31 as unusable and /32 as a single host route.
§3Examples of Subnetting Blocks
The RFC gives example tables for starting blocks of /16 and /24. Each example shows how many bits are used to divide the block, the resulting mask, the effective number of subnets, and the effective hosts per subnet.
The examples exclude subnet addresses with all zeroes or all ones, and also exclude host addresses with all zeroes or all ones. The counts in the examples follow those exclusions.
The examples show how changing the mask divides a starting block into more or fewer subnets. Their counts leave out subnet and host values that are all zeroes or all ones.