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IP Subnetting Summary Tables For Class A, Class B and Class C Networks
(Page 1 of 4)
Since there are only a few options
for how to subnet each of Class A, Class B and Class C networks, I have
listed the options for each class in three summary tables below: Table 52,
and Table 54.
These tables can help you quickly decide
how many bits to use for subnet ID and host ID,
and then what the subnet mask is for their selection. They also summarize
nicely what we've discussed in this section.
Each row of each table shows one
possible subnetting option for that class, including the number of bits
for each of the subnet ID and host ID, and the number of subnets and
hosts based on the number of bits. I then show the subnet mask in binary
and decimal form, as well as in CIDR
notation. Finally, I include the formula
for calculating the addresses for each subnet under each of the options.
Notes on the Three Subnetting Summary Tables
A few additional explanatory notes
are in order regarding these tables:
- The values for the number of subnets per network
assume that the all-zeroes and all-ones subnets are allowed. If
not, you must subtract 2 from those figures;
this also means that the option using only one bit for the subnet ID
becomes invalid, and the subnet address formulas no longer work as shown.
- The number of hosts per subnet does
exclude the all-zeroes and all-ones cases, so it is two to the power
of the number of host ID bits, less two.
- The first row of each table shows the default
case where the number of subnet bits is zero, and thus the subnet mask
is the default
subnet mask for the class.
- In the subnet mask for all options but the default,
I have highlighted the portion of the subnet mask corresponding to the
subnet ID, for clarity. This has been done for each individual bit of
the binary mask, and for each octet in the dotted decimal representation
of the mask where part of the subnet ID is found.
- You will see that not all of the divisions make
a great deal of sense in the real world
though you might be surprised.
For example, at first glance it seems silly to think that we might want
to assign 14 bits of a Class B host ID to the subnet ID and leave 2
bits for the host IDwhat sort of real network has 16,384 subnets
with 2 hosts on each? Yet some larger Internet service companies may
indeed require thousands of tiny subnets when setting up connections
between routers, or between their core network and their customers.
- The subnet address formulas in the last column
of each table show the address for subnet #N (numbering from zero up
to one less than the maximum number of subnets).See
the end of step #4 in the step-by-step subnetting section
for a full explanation of how these formulas work.
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Version 3.0 - Version Date: September 20, 2005
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