We can’t emphasize enough the importance of being able to calculate subnets on the fly when working with Windows servers and clients, particularly for designing your Active Directory site structure (also something you will get hammered on in MS exam 70-642: MCTS: Windows Server Network Infrastructure, Configuring). 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.For people who work with IP addressing and routing as part of their jobs, subnetting (breaking a larger IP network into separated and smaller networks) or supernetting (combining two or more networks or subnets together) 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. In this post, we will address the more complicated subnetting known as extended subnetting. Instead of using the basic classfulsubnet masks (255.0.0.0, 255.255.0.0 and 255.255.255.0), we will explore the use of extended subnet masks.
useful.For people who work with IP addressing and routing as part of their jobs, subnetting (breaking a larger IP network into separated and smaller networks) or supernetting (combining two or more networks or subnets together) 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. In this post, we will address the more complicated subnetting known as extended subnetting. Instead of using the basic classfulsubnet masks (255.0.0.0, 255.255.0.0 and 255.255.255.0), we will explore the use of extended subnet masks.
SOME RULES TO REMEMBER
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 11111111.11111111.11000000.00000
000. Notice that we have used the first 18 of the 32 bits for the
subnet mask. This is where the /18 notation originates.
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)
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 11111111.11111111.11000000.00000
000. Notice that we have used the first 18 of the 32 bits for the
subnet mask. This is where the /18 notation originates.
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)
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