2002-08-07 16:28:04 +02:00
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<title>Subnet Masks</title>
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2002-08-22 23:33:54 +02:00
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2002-08-07 16:28:04 +02:00
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2002-08-22 23:33:54 +02:00
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<table border="0" cellpadding="0" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%" id="AutoNumber1" bgcolor="#400169" height="90">
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<td width="100%">
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<h1 align="center"><font color="#FFFFFF">Subnet Masks/VLSM Notation</font></h1>
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2002-08-07 16:28:04 +02:00
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<p align="left">IP addresses and subnet masks are 32-bit numbers. The notation
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w.x.y.z refers to an address where the high-order byte has value "w", the next
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byte has value "x", etc. If we take 255.255.255.0 and express it in
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hexadecimal,
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we get:</p>
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<blockquote>
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<p align="left">FF.FF.FF.00</p>
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</blockquote>
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<p align="left">or looking at it as a 32-bit integer</p>
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<blockquote>
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<p align="left">FFFFFF00</p>
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</blockquote>
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<p align="left">Each "F" represents the bit pattern "1111" so if we look at the
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number in binary, we have:</p>
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<blockquote>
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<p align="left">11111111111111111111111100000000</p>
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</blockquote>
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<p align="left">Counting the leading "1" bits, we see that there are 24 -- /24
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in VLSM notation.</p>
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<p align="left">It is handy to remember that the size of the subnet can be
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obtained by subtracting the number of consecutive leading "1" bits from 32 and
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raising 2 to that power. In the above case, 32 - 24 = 8 and 2 ** 8 = 256
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addresses. Remember that the number of usable addresses is two less than that
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(254) because the first and last address in the subnet are reserved as the
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sub-network and broadcast addresses respectively.</p>
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<p align="left">The size of a subnet can be any power of two so long as the
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address of the subnet is a multiple of it's size. For example, if you want a
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subnet of size 8, you could choose 192.168.12.8/29 (8 = 2 ** 3 and 32 - 3 = 29).
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The subnet mask would be:</p>
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<blockquote>
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<p align="left">11111111111111111111111111111000 = FFFFFFF8 = 255.255.255.248.</p>
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</blockquote>
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<p align="left">This subnet would have 6 usable addresses: 192.168.12.9 -
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192.168.12.14.</p>
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<p align="left">You will still hear the terms "Class A network", "Class B
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network" and "Class C network". In the early days of IP, sub-networks only came
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in three sizes:</p>
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<blockquote>
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<p align="left">Class A - Subnet mask 255.0.0.0, size = 2 ** 24</p>
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<p align="left">Class B - Subnet mask 255.255.0.0, size = 2 ** 16</p>
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<p align="left">Class C - Subnet mask 255.255.255.0, size = 256</p>
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</blockquote>
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<p align="left">The class of a network was determined by the value of the high
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order byte of its address so you could look at an IP address and immediately
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determine the associated subnet mask. </p>
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<p align="left">As the internet grew, it became clear that such a gross
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partitioning of the 32-bit address space was going to be very limiting (early
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on, large corporations and universities were assigned their own class A
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network!). It was then that VLSM was devised -- today, any system that you are
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likely to work with understands VLSM and Class-based subnetworking is largely a
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thing of the past.</p>
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<p align="left"><font size="2">Last updated
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7/15/2002 - <a href="support.htm">Tom
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Eastep</a></font></p>
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<p><a href="copyright.htm"><font size="2">Copyright 2002 Thomas M. Eastep</font></a></p>
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2002-08-22 23:33:54 +02:00
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2002-08-07 16:28:04 +02:00
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