Quick lesson on configuring subnet addresses.
- By Bill Boswell
- July 16, 2003
I am currently studying for my MCSE but
am having a hard time comprehending subnetting. Do you know of any books,
articles, etc. I can read on this subject that will explain it in simple
I feel your pain, Frank. I really do. We sysadmins don't deal with networking
every minute of the day like our colleagues in the network group.
Here's one way of looking at the problem. A 32-bit IP address is divided
into four 8-bit sections (octets) represented by their decimal equivalent.
You can use the Scientific mode of the Windows calculator to determine
the binary number for an octet. For example, the address 192.168.0.1 is
really this 32-bit number:
11000000 10101000 00000000
You can do the same calculation on paper. Scribble a quick chart of powers
of 2 and their decimal equivalents:
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The top four bits of the top octet determines the network class designation.
If the top bit of the top octet is 0, the address is Class A. So, if a
stranger comes up to you in the mall and asks the class of the address
188.8.131.52, you'll tell him that 83 is really 64+16+1, which yields binary
01010001, so the first bit is zero and therefore the address is Class
The upper octet bits of a Class A address can lie in a range of 00000001
to 01111111, so the Class A address range spans 1 to 64+32+16+8+4+2+0,
or 126. (The actual endpoint, 127, is a special case. It's the loopback
address, the address that represents the local adapter. It cannot be a
The standard Class A subnet mask is 255.0.0.0, meaning that all bits
in the upper octet determine the network address while the remaining bits
in the other three octets determine the host address. So an address of
184.108.40.206 with a subnet mask of 255.0.0.0 has a network address of 83
and a host address of 128.1.37. The binary mask looks like this:
If the top bit of the first octet is set to 1 and the second bit is set
to 0, the address is Class B. This yields a span of 10000000 to 10111111,
or 128 to 191.
The default subnet mask for Class B is 255.255.0.0, meaning that the
first 16 bits of the address determine the network address. Take an address
of 220.127.116.11, for example.
If the top two bits are set to 1 and the third bit is set to 0, the address
is Class C. This yields a span of binary 11000000 11011111 or 192 to 223.
The class D address range, 224 (11100000) to 239 (11101111) is used for
multicasting. The class E address range starting at 240 (11110000) is
As you know, it is possible to “steal” bits from the host
address space in the lower three octets to build a larger network address
space for a given address class. For example, a Class C address such as
18.104.22.168 with a subnet mask of 255.255.224 “steals” three
bits from the host ID in the final octet. This leaves five bits for host
To determine the number of networks and hosts available for a given subnet
mask, jot down the power of 2 chart and put in the bits associated with
the mask. In the example, 224 represents three bits.
The total of available networks represents the sum of the powers of two
in the upper bits of the mask’s final octet. In the example, this
is 1+2+4, or 7. The total of available hosts represents the sum of the
powers of two in the lower bits of the mask. In the example, this is 1+2+4+8+16
You have to do this sort of arithmetic in a lot of different examples
to get the hang of it. It’s a little like learning French verbs.
For more references, I think Brian Komar does a good job of explaining
subnets in his book Teach
Yourself TCP/IP Networking in 21 Days (SAMS) and I like the examples
you’ll find in Microsoft
Windows Server 2003 TCP/IP Protocols and Services Technical Reference
by Joseph Davies and Thomas Lee (Microsoft Press).
A Hat in Exchange for Feedback
In response to last week's column, "Finding
Users on the Network," Adrian F. DickReiter, MCSE+I, MCSA, of
San Antonio, Texas, writes:
Boswell, try using the Psloggedon utility that can be found at http://www.sysinternals.com/ntw2k/freeware/pstools.shtml.
With it you type in a username and it systematically goes through all
computers on the network to look for a particular user. I've never found
a way to capture this data so as to isolate what computers a user is
logged onto, but I'm sure there's a way to do it—maybe even something
so simple as piping it to a Notepad file might work. It's a really great
Thanks, Adrian. For that advice, we're sending you an MCPmag.com