TCP/IP Addressing and Subnetting
Class A
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Highest bit of the first octet is (0)
First 8 bits are normally allotted network and the remaining 24 bits are allotted to hosts
Class range is 1 - 126. (127 is reserved for loopback)
Default Mask is 8 bits
126 possible Class A networks
16,777,213 hosts in each network
Class B
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Highest bit of the first octet is (10)
First 16 bits are normally allotted network and the remaining 16 bits are allotted to hosts
Class range is 128 – 191
Default mask is 16 bits
16,383 possible Class B networks
65,533 hosts in each network
Class C
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Highest bit of the first octet is (110)
First 24 bits are normally allotted network and the remaining 8 bits are allotted to hosts
Class range is 192 – 223
Default mask is 24 bits
2,097,151 possible Class C networks
254 hosts in each network
Bitwise ANDing
A bitwise AND takes two binary representations of equal length and performs the logical AND
operation on each pair of corresponding bits. The result in each position is 1 if the first bit is 1 and
the second bit is 1; otherwise, the result is 0. In this, we perform the multiplication of two bits:
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1×0=0
1×1=1
0x0=0
The number of networks can be increased by adding bits to the default subnet mask:
Class C
Bits
Added
Notation
Mask
Subnets
Hosts
0
/24
255.255.255.0
1
256
1
/25
255.255.255.128
2
128
2
/26
255.255.255.192
4
64
3
/27
255.255.255.224
8
32
4
/28
255.255.255.240
16
16
5
/29
255.255.255.248
32
8
6
/30
255.255.255.252
64
4
7
/31
255.255.255.254
128
2
8
/32
255.255.255.255
256
1
Class B
Bits
Added
Notation
Mask
Subnets
Hosts
0
1
2
3
4
5
6
7
8
/16
/17
/18
/19
/20
/21
/22
/23
/24
255.255.0.0
255.255.128.0
255.255.192.0
255.255.224.0
255.255.240.0
255.255.248.0
255.255.252.0
255.255.254.0
255.255.255.0
1
2
4
8
16
32
64
128
256
65,536
32,768
16,384
8,192
4,096
2,048
1,024
512
256
Class A
Bits
Added
Notation
Mask
Subnets
Hosts
0
/8
255.0.0.0
1
16,777,216
1
/9
255.128.0.0
2
8,388,608
2
/10
255.192.0.0
4
4,194,304
3
/11
255.224.0.0
8
2,097,152
4
/12
255.240.0.0
16
1,048,576
5
/13
255.248.0.0
32
524,288
6
/14
255.252.0.0
64
262,144
7
/15
255.254.0.0
128
131,072
8
/16
255.255.0.0
256
65,536