Tuesday, 7 August 2012

CCNA

Subnetting Table
Use this table to help you calculate Subnet Masks, available hosts per subnet or number of subnets per class. The table deals with Subnetting of only one octet.

# of Masked bits
# of Non-masked bits
Bit pattern
# of subnets =2^M
What will the new subnet mask be?
# of hosts per subnet (C Class)
=[(2^
N)-2]
# of hosts per subnet (B Class)
=[(2^
N)-2]
# of hosts per subnet (A Class)
=[(2^
N)-2]
None
0
8
00000000
0
0
254
65,534
~16 Million
1
1
1
7
10000000
2
128
126
32,766
~8 Million
2
2
2
6
11000000
4
192
62
16,382
~4 Million
3
4
3
5
11100000
8
224
30
8,190
~2 Million
4
8
4
4
11110000
16
240
14
4,094
~1 Million
5
16
5
3
11111000
32
248
6
2,046
~520,000
6
32
6
2
11111100
64
252
2
1,022
~260,000
7
64
7
1
11111110
128
254
-
510
~130,000
8
128
8
0
11111111
256
255
-
254
~65,000

M=Number of Masked bits
N=Number of Non-masked bits - Remember that in Class A and B networks you have other octets besides the one you're subnetting. Therefore in a Class A network instead of saying 7 (for example) you must say 7+8+8. In a Class B network instead of saying 5 (for example again, duh!) you must say 5+8. Only in Class C networks does the N value remain the same.

Starting bits for the first octet
How many networks per Class?
How many hosts per network?
Range of the first octet

Class A
0
126
(2^24)-2=~16 Million
1-126
127=Loopback
Class B
10
2^14=16384
(2^16)-2=~65000
128-191

Class C
110
2^21=~2 Million
(2^8)-2=254
192-223

Class D
1110
-
-
224-239

Class E
1111
-
-
240-255


How many potential IP addresses do we have?
126 networks of 16,777,214 hosts each = 2,113,928,964
16,384 networks of 65,534 hosts each = 1,073,709,056
2,097,152 networks of 254 hosts each = 532,676,608
Total number of hosts = 3,720,314,628
Note: In Windows NT 4.0 you had to subtract 2 from the total number of available subnets. For example, if you wanted 2 subnets, you had to figure the number you wanted plus 2 - calculate like you wanted 4, i.e. the result would have been 192. If you wanted 8 subnets, you had to calculate like you wanted 10, i.e. the result would have been 240 instead of 224. With Windows 2000 and beyond Subnetting you do not have to do that anymore.

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