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We can’t emphasize enough the importance of being able to calculate subnets on the fly when working with Windows 2000, particularly for designing your Active Directory site structure (also something you will get hammered on in MS exam 70-216 and 70-221).
If you know the principles and theory behind subnetting and can
convert binary numbers to decimal and decimal to binary, you may find this very short summary of how to perform subnetting useful. If you are not at the stage where you are comfortable with either binary numbers or the principles behind subnetting, you might want to read over our two-part series, Learn to Subnet (URLs at bottom of page).
For people who work with IP addressing and routing as part of
their jobs, subnetting or supernetting becomes second nature.
For them, it is a fairly easy thing to calculate subnets. A
lot of people who are new to networking or relatively
inexperienced with TCP/IP often and wrongly think that performing subnetting is a mysterious and difficult task. I remember the frustration I felt many years ago when, as an inexperienced network admin, I could not get anyone to give me a good explanation of subnetting. Some of the more experienced network administrators, in fact, refused to explain it to me. I don't know if they were protecting their jobs or what, but it did seem to me at the time that they were going out of their way to create a false aura of mystery and difficulty concerning subnetting.
The truth is this: calculating subnets is fairly easy and doesn't require that you memorize anything except a few general principles and know how to work with binary numbers. The purpose of this short explanation is to provide you with the minimum amount of information you have to carry around in your head in order to calculate subnet (or supernet) masks in any situation, an exam, your job, etc.
Some rules to remember:
1) The network portion of the IP address described by the
extended subnet mask cannot be expressed as all 0s or all
1s. (The validity of this rule depends on a number of
factors such as the type of hardware and the routing
protocol in use.) So, for example, let's assume we have
extended a Class B network by borrowing two bits from the
host portion of the address thus: 172.16.0.0/18. The
subnet mask is 255.255.192.0 or
11111111.11111111.11000000.00000000.
Any combination of 0s and 1s in the 3rd octet could possibly comprise a network ID. However, following our rule of not allowing all 1s or all 0s in the positions covered by the extended subnet mask, we get the following valid network IDs in the 3rd octet:
00000000 = 0 Not a valid network ID (all 0s)
01000000 = 64 Valid network ID (172.16.64.0)
10000000 = 128 Valid network ID (172.16.128.0)
11000000 = 192 Not a valid network ID (all 1s)
2) The host portion of the IP address cannot be expressed as
all 0s or all 1's. All 0s in the host portion of the
address indicates the Network ID. All 1's in the host
portion of the address indicates the address for the
broadcast on the subnet. Therefore, using our example
above, here are the valid host IDs for our two valid
subnets 172.16.64.0/18 and 172.16.128.0/18:
172.16.64.1/18 – 172.16.127.254/18
172.16.128.1/18 – 172.16.191.254/18
To understand the rule more clearly, convert the host
addresses to binary:
64 1 127 254
x.y.01000000.00000001 – x.y.01111111.11111110
128 1 191 254
x.y.10000000.00000001 – x.y.10111111.11111110
3) To calculate the number of hosts or the number of networks
for a given subnet mask, use the following formula:
2^(number of bits used) – 2 = number of networks/hosts
(The '^' means 'raised to the power of'.)
So, given the fact that we have borrowed 2 bits to extend
our network, we have 2^2 – 2 = 2 networks (remember, we are
excluding the all 1 and all 0 networks).
We have 14 bits left to describe the hosts on each network;
therefore we have 2^14 – 2 = 16,382 hosts per network
(remember we are excluding host addresses with all 0s or
all 1s).
4) The value of the lowest order bit in the subnet mask tells
you two things: a) it tells you the first network ID for
the subnets created by the extended subnet mask, b) it
tells you the value that you can add to one network ID to
arrive at the next possible network ID.
Let's take our example extended subnet mask of
255.255.192.0. The 3rd octet expressed in binary with the
decimal equivalents is
128 64 32 16 8 4 2 1
1 1...
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