IPv4 VLSM Table: Variable Length Subnet Table For IPv4
A reference table of all possible variable-length subnet masks for IPv4 addresses.
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
AI ELI5
On this page
§1Variable Length Subnetting
IPv4 addresses can be divided into a network portion and a host (computer) portion using a subnet mask. Variable Length Subnet Masking (VLSM) allows administrators to use different mask sizes on different parts of the network, making efficient use of address space.
Each mask can be written in three ways: as a hexadecimal value (like FF.FF.FF.00), as CIDR notation (like /24, meaning the first 24 bits are the network), or as a dotted decimal (like 255.255.255.0).
An Internet address is two parts: which network and which computer on that network. A mask says where to split between them. Different masks let you split at different places.
§2The Subnet Table
The RFC lists all 32 possible subnet masks (from /1 to /32), each with its value in all three notations. For example, the /24 mask is written as FF.FF.FF.00 in hex, 255.255.255.0 in decimal, and provides 256 addresses per subnet (though the all-zeros and all-ones addresses are typically reserved).
Each mask also shows its classful equivalent (a class A, B, or C network from the older routing scheme). This helps administrators understand how masks relate to older network structures and the number of usable host addresses in each subnet.
The table shows: for each mask size, what it looks like in different notations and how many computers can be on a subnet with that mask.
§3Subdividing Networks
The RFC includes examples of how to use VLSM to subdivide common network blocks. For a /16 network (a classful Class B), administrators can add 2 to 14 more bits to the mask and create different numbers of subnets with different numbers of hosts. For a /24 network (a classful Class C), they can add 2 to 6 more bits.
For instance, within a /16 block, using a /24 mask creates 254 subnets, each with 254 hosts; using a /30 mask creates 16,382 subnets, each with 2 hosts (useful for point-to-point links).
Depending on how you split an address block, you can make many small networks or a few big ones. The table shows the math for common block sizes.