Subnetting Overview
Last Updated: 08 Feb 2004
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*** PLEASE NOTE: Link(s), If Provided, May Be Wrapped ***
This document provides a basic overview on IP Subnetting.
To properly calculate IP subnets and determine the correct
number of networks or hosts, you would be well advised to
use base2 numbering, as opposed to base10 numbering.
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USING CLASSFUL IP ADDRESSING
==============================
CLASS A .................. 1.x.x.x to 126.x.x.x
CLASS B .................. 128.x.x.x to 191.x.x.x
CLASS C .................. 192.x.x.x to 223.x.x.x
DEFAULT SUBNET MASKS
CLASS A = 255.0.0.0
11111111.00000000.00000000.00000000
nnnnnnnn.HHHHHHHH.HHHHHHHH.HHHHHHHH
/8
CLASS B = 255.255.0.0
11111111.11111111.00000000.00000000
nnnnnnnn.nnnnnnnn.HHHHHHHH.HHHHHHHH
/16
CLASS C = 255.255.255.0
11111111.11111111.11111111.00000000
nnnnnnnn.nnnnnnnn.nnnnnnnn.HHHHHHHH
/24
The portion of an IP address which corresponds to the
ONEs (or the "n") in the subnet mask, is the Network
Address. The portion covered by the ZEROs (or the "H")
is the Host Address.
An IP address of all ZEROs for the Host Address represents
the Network Name (i.e. 192.168.99.0 in a Class C network)
An IP address of all ONEs for the Host Address represents
the Broadcast Address (i.e. 192.168.99.255)
IP Addresses can also be denoted in slash notation to
represent the subnet mask. For example, 192.168.99.5/20
is the same as:
IP ...... 192.168.99.5
Mask .... 255.255.240.0 (11111111.11111111.11110000.00000000)
Taking an address like 10.100.31.69, the network number
and the host number depend on the subnet mask:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NOTE: In classful addressing, 10.100.31.69 would always
be considered a "Class A" address. However, for
purposes of simplifying this example, we're going
to use the same IP address for all 3 classes.
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Class A:
--> Network Number ... 10
--> Host Number ...... 100.31.69
Class B:
--> Network Number ... 10.100
--> Host Number ...... 31.69
Class C:
--> Network Number ... 10.100.31
--> Host Number ...... 69
===========================================================
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USING CLASSLESS IP ADDRESSING
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Now, once you branch away from the default subnet masks,
and your standard Class A/B/C networks, you have to do a
bit more work to derive network/host numbers:
IP Address ....... 10.100.31.69 (00001010.01100100.00011111.01000101)
Subnet Mask ...... 255.255.240.0 (11111111.11111111.11110000.00000000)
Network Number ... 10.100.16 (00001010.01100100.00010000.00000000)
Host Number ...... 15.69 (00000000.00000000.00001111.01000101)
The 14 networks available to the above subnet mask (/20) are:
Network/Host (nnnnnnnn.nnnnnnnn.nnnnHHHH.HHHHHHHH)
10.100.16.0 (00001010.01100100.00010000.00000000)
10.100.32.0 (00001010.01100100.00100000.00000000)
10.100.48.0 (00001010.01100100.00110000.00000000)
10.100.64.0 (00001010.01100100.01000000.00000000)
10.100.80.0 (00001010.01100100.01010000.00000000)
10.100.96.0 (00001010.01100100.01100000.00000000)
10.100.112.0 (00001010.01100100.01110000.00000000)
10.100.128.0 (00001010.01100100.10000000.00000000)
10.100.144.0 (00001010.01100100.10010000.00000000)
10.100.160.0 (00001010.01100100.10100000.00000000)
10.100.176.0 (00001010.01100100.10110000.00000000)
10.100.192.0 (00001010.01100100.11000000.00000000)
10.100.208.0 (00001010.01100100.11010000.00000000)
10.100.224.0 (00001010.01100100.11100000.00000000)
You'll notice that the network numbers are covered ONEs
(or the "n") of the subnet mask.
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NOTE: As with Classful Addressing, network numbers with
all ONEs or all ZEROs are generally not used.
By default, this rules out the following networks
as valid for this subnet mask:
• 10.100.0.0
• 10.100.240.0
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SUBNETTING TUTORIALS
• http://www.dalantech.com/ubbthreads/showflat.php?Number=33050
• http://www.more.net/technical/netserv/tcpip/ipaddressing.html
• http://www.telusplanet.net/public/sparkman/netcalc.htm
• http://www.ralphb.net/IPSubnet/
• http://www.inlink.com/~benson/files/tech/subnet1.html
• http://www.cisco.com/warp/public/701/3.html
• http://www.learntosubnet.com/
• http://www.febo.com/ipaddr/tutorial.html
• http://www.sinclair.org.au/keith/networking/subnet1.html
• http://www.cis.ohio-state.edu/htbin/rfc/rfc1878.html
• http://www.net3group.com/ipcalc.asp
• http://www.3com.com/nsc/501302s.html
• http://KB.UltraTech-llc.com/Docs/?File=IPAddressing-SOHO.PDF
PERSONAL NOTES
• There's a whole lot to know about in the land of
networking, with DHCP, DNS and Subnetting being the
most common issues.
