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IP Subnetting Step #3: Determining The Custom Subnet Mask
(Page 2 of 2)
Class B Custom Subnet Mask Calculation Example
Now, let's do the same example with
our Class B network (126.96.36.199) with 5 bits for the subnet ID (with
a bit less narration this time; see Figure 77):
Figure 77: Determining The Custom Subnet Mask for A Class B Network
- Determine Default Subnet Mask:
For Class B, the subnet mask is 255.255.0.0. In binary, this is:
11111111 11111111 00000000
- Change Left-Most Zeroes To Ones
For Subnet Bits: We have decided to use 5 bits for the subnet ID,
so, we change the 5 left-most 0 bits from a 0 to 1, shown highlighted
below, to give us our binary custom subnet mask:
11111111 11111111 11111000
Determining The Custom Subnet Mask Using Subnetting Tables
- Convert Subnet Mask To Dotted Decimal
Notation: We take each of the octets in the subnet mask and convert
it to decimal, to give us a custom subnet mask of 255.255.248.0
- Express Subnet Mask In Slash
Notation: We can express the subnet mask 255.255.248.0 as
/21, since it is 21 ones followed by 11 zeroes. In other
words, its prefix length is 21.
Now, what could be easier than that?
Well, you could simply refer
to the subnetting summary tables. Find
the table for the appropriate class, and then find the row that you
selected in the previous step that matches the number of subnet ID bits
you want to use. You can see the matching subnet mask right there.
(Hey, it's good to know how to do
it yourself! You may not always have tables to refer to!)
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Version 3.0 - Version Date: September 20, 2005
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