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192.168.1.0 with mask 255.255.255.128 is subnet
A
itself, and 192.168.1.127 with mask 255.255.255.128 is its broadcast
address. Therefore, the lowest IP address that can be assigned to an actual host for subnet
A
is 192.168.1.1 and the
highest is 192.168.1.126.
Similarly, the host ID range for subnet
B
is 192.168.1.129 to 192.168.1.254.
Example: Four Subnets
The previous example illustrated using a 25-bit subnet mask to divide a 24-bit address into two subnets. Similarly, to divide
a 24-bit address into four subnets, you nee
d to ―borrow‖ two host ID bits to give four possible combinations (00, 01, 10 and
11). The subnet mask is 26 bits (11111111.11111111.11111111.
11
000000) or 255.255.255.192.
Each subnet contains 6 host ID bits, giving 2
6
- 2 or 62 hosts for each subnet (a host ID of all zeroes is the subnet itself, all
ones is the subnet‘s broadcast address).
Subnet 1
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address (Decimal)
192.168.1.
0
IP Address (Binary)
11000000.10101000.00000001.
00
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.0
Lowest Host ID: 192.168.1.1
Broadcast Address:
192.168.1.63
Highest Host ID: 192.168.1.62
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103
Subnet 2
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address
192.168.1.
64
IP Address (Binary)
11000000.10101000.00000001.
01
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.64
Lowest Host ID: 192.168.1.65
Broadcast Address:
192.168.1.127
Highest Host ID: 192.168.1.126
Subnet 3
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address
192.168.1.
128
IP Address (Binary)
11000000.10101000.00000001.
10
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.128
Lowest Host ID: 192.168.1.129
Broadcast Address:
Highest Host ID: 192.168.1.190
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104
192.168.1.191
Subnet 4
IP/SUBNET MASK
NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address
192.168.1.
192
IP Address (Binary)
11000000.10101000.00000001.
11
000000
Subnet Mask (Binary)
11111111.11111111.11111111.
11
000000
Subnet Address:
192.168.1.192
Lowest Host ID: 192.168.1.193
Broadcast Address:
192.168.1.255
Highest Host ID: 192.168.1.254
Example: Eight Subnets
Similarly, use a 27-bit mask to create eight subnets (000, 001, 010, 011, 100, 101, 110 and 111).
The following table shows IP address last octet values for each subnet.
Eight Subnets
SUBNET
SUBNET
ADDRESS
FIRST ADDRESS
LAST
ADDRESS
BROADCAST
ADDRESS
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105
1
0
1
30
31
2
32
33
62
63
3
64
65
94
95
4
96
97
126
127
5
128
129
158
159
6
160
161
190
191
7
192
193
222
223
8
224
225
254
255
Subnet Planning
The following table is a summary for subnet planning on a network with a 24-bit network number.
24-bit Network Number Subnet Planning
NO. “BORROWED”
HOST BITS
SUBNET MASK
NO.
SUBNETS
NO. HOSTS PER
SUBNET
1
255.255.255.128 (/25)
2
126
2
255.255.255.192 (/26)
4
62
3
255.255.255.224 (/27)
8
30
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106
4
255.255.255.240 (/28)
16
14
5
255.255.255.248 (/29)
32
6
6
255.255.255.252 (/30)
64
2
7
255.255.255.254 (/31)
128
1
The following table is a summary for subnet planning on a network with a 16-bit network number.
16-bit Network Number Subnet Planning
NO.
“BORROWED”
HOST BITS
SUBNET MASK
NO.
SUBNETS
NO. HOSTS PER
SUBNET
1
255.255.128.0 (/17)
2
32766
2
255.255.192.0 (/18)
4
16382
3
255.255.224.0 (/19)
8
8190
4
255.255.240.0 (/20)
16
4094
5
255.255.248.0 (/21)
32
2046
6
255.255.252.0 (/22)
64
1022
7
255.255.254.0 (/23)
128
510
8
255.255.255.0 (/24)
256
254
9
255.255.255.128 (/25)
512
126
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