Subnetting Shortcuts
Class C
200.4.5.0 Borrow
3 bits Subnet Mask = 255.255.255.224
Number of
networks created 23 = 8
Number of useable networks created 23 = 8 – 2 = 6
Number of
host per network 25 = 32
Number of useable host per network 25 = 32 – 2 = 30
The
increment for each network is 32
If you
need to determine the network number of subnet 6,
multiply 6 X 32 = 192.
The subnet 6 network address would be 200.4.5.192
In simple
terms, multiply the increment by the number of the subnet
you wish to find.
Class B – borrowing
in the 3rd octet
150.4.0.0 Borrow
3 bits Subnet Mask = 255.255.224.0
Number of
networks created 23 = 8
Number of useable networks created 23 = 8 – 2 = 6
Number of
host per network 25 X 256 = 8192
The
increment for each network is 32 in the 3rd octet
(the number of unmasked bits in the 3rd octet is 5, this is
the octet we borrowed from)
If you
need to determine the network number of subnet 6,
multiply 6 X 32 = 192.
The subnet 6 network address would be 150.4.192.0
If you
are borrowing in the 3rd octet, just ignore the 4th
octet to determine your network numbers.
Class B – borrowing
all the 3rd octet
150.4.0.0 Borrow
8 bits Subnet Mask = 255.255.255.0
Number of
networks created 28 = 256
Number of useable networks created 28 = 256 – 2 = 254
Number of
host per network 20 X 256 = 256
The
increment for each network is 256 in the 3rd octet.
(The number of unmasked bits in the 3rd octet is 0, this is
the octet we borrowed from.
This means that the value of the 3rd octet increases by one each
time.)
If you
need to determine the network number of subnet 26, simply
insert that number into the 3rd octet slot.
The subnet 26 network address would be 150.4.26.0
If you
need to determine the network number of subnet 100, simply
insert that number into the 3rd octet slot.
The subnet 100 network address would be 150.4.100.0
Class B – borrowing
in the 4th octet
150.4.0.0 Borrow
11 bits Subnet Mask = 255.255.255.224
Number of
networks created 211 = 2048 (or 23 X 256 = 2048)
Number of
useable networks created 211 = 2048 – 2 = 2046
Number of
host per network 25 = 32
The
increment for each network is 32 in the 4th octet.
(The number of masked bits in the 4th octet is 3, the number of
unmasked bits in the 4th octet is 5, this is the last octet we
borrowed from.
This means that the value of the 4th octet increases by the
increment value.)
If you
need to determine the network number of subnet 325, do the following:
Divide
the desired subnet by 2 raised to the masked bits in the 4th octet
325 / 23
or 325 / 8 = 40 remainder 5
The 40 is
the value of the 3rd octet -- 150.4.40.?
To find
the value of the 4th octet multiply the remainder (5) times the
increment (32)
32 X 5 =
160 This is the value of the 4th octet
The
subnet 325 network address would be 150.4.40.160
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If
you need to determine the network number of subnet 40, do the
following:
Divide
the desired subnet by 2 raised to the masked bits in the 4th octet
40 / 23
or 40 / 8 = 5 remainder 0
The 5 is
the value of the 3rd octet -- 150.4.5.?
To find
the value of the 4th octet multiply the remainder (0) times the
increment (32)
32 X 0 =
0 This is the value of the 4th octet
The
subnet 40 network address would be 150.4.5.0
Another Method
Class B – borrowing in
the 4th octet
150.4.0.0 Borrow
11 bits Subnet Mask = 255.255.255.224
Number of
networks created 211 = 2048 (or 23 X 256 = 2048)
Number of
useable networks created 211 = 2048 – 2 = 2046
Number of
host per network 25 = 32
The
increment for each network is 32 in the 4th octet. (The number of
masked bits in the 4th octet is 3, the number of unmasked bits in
the
4th octet is 5, this is the last octet we borrowed from.
This means that the value of the 4th octet increases by the
increment value.)
Another way to determine the Network Number of a Subnet:
