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Notation
Since the mask is always a continuous number of ones beginning from the left,
followed by a continuous number of zeros for the remainder of the 32 bit mask,
you can simply specify the number of ones instead of writing the value of each
octet. This is usually specified by writing a “/” followed by the number of bits in
the mask after the address.
For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with subnet mask
255.255.255.128.
The following table shows some possible subnet masks using both notations.
Subnetting
You can use subnetting to divide one network into multiple sub-networks. In the
following example a network administrator creates two sub-networks to isolate a
group of servers from the rest of the company network for security reasons.
In this example, the company network address is 192.168.1.0. The first three
octets of the address (192.168.1) are the network number, and the remaining
octet is the host ID, allowing a maximum of 2
8
– 2 or 254 possible hosts.
Table 82
Alternative Subnet Mask Notation
SUBNET MASK
ALTERNATIVE
NOTATION
LAST OCTET
(BINARY)
LAST OCTET
(DECIMAL)
255.255.255.0
/24
0000 0000
0
255.255.255.128
/25
1000 0000
128
255.255.255.192
/26
1100 0000
192
255.255.255.224
/27
1110 0000
224
255.255.255.240
/28
1111 0000
240
255.255.255.248
/29
1111 1000
248
255.255.255.252
/30
1111 1100
252
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The following figure shows the company network before subnetting.
Figure 118
Subnetting Example: Before Subnetting
You can “borrow” one of the host ID bits to divide the network 192.168.1.0 into
two separate sub-networks. The subnet mask is now 25 bits (255.255.255.128 or
/25).
The “borrowed” host ID bit can have a value of either 0 or 1, allowing two
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The following figure shows the company network after subnetting. There are now
two sub-networks,
A
and
B
.
Figure 119
Subnetting Example: After Subnetting
In a 25-bit subnet the host ID has 7 bits, so each sub-network has a maximum of
2
7
– 2 or 126 possible hosts (a host ID of all zeroes is the subnet’s address itself,
all ones is the subnet’s broadcast address).
192.168.1.0 with mask 255.255.255.128 is subnet
A
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
Similarly, the host ID range for subnet
B
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 need 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.
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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).
Table 83
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
Broadcast Address:
192.168.1.63
Highest Host ID: 192.168.1.62
Table 84
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
Table 85
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
Broadcast Address:
192.168.1.191
Table 86
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
Broadcast Address:
192.168.1.255
Highest Host ID: 192.168.1.254
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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.
Subnet Planning
The following table is a summary for subnet planning on a network with a 24-bit
network number.
The following table is a summary for subnet planning on a network with a 16-bit
network number.
Table 87
Eight Subnets
SUBNET
SUBNET
ADDRESS
FIRST ADDRESS
LAST
ADDRESS
BROADCAST
ADDRESS
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
Table 88
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
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
Table 89
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
19216811.live