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Class A Subnetting
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This material was developed with funding from the
National Science Foundation under Grant # DUE 1601612
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Default Subnet Mask
Range of a Class A Network Addresses 1.x.x.x – 126.x.x.x
126 Networks
A default Class A subnet mask uses the first octet to identify the network portion of a Class A IP address and the last three octets are the host portion of the address.
16,777,216 Hosts Per Network
Borrowing Bits
The number of bits borrowed determines the new subnet mask. For example, borrowing 12 bits from a Class A network results in the following:
Default Class A subnet mask = 255.0.0.0 The new subnet mask with 12 borrowed bits = 255.255.240.0
255.255.240.0 represented as a bitwise mask = /20
Note: The bitwise mask value equals the default subnet mask bits (8) plus the borrowed bits (12).
You can write 12.0.0.0 255.255.240.0 as 12.0.0.0 /20
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Drag the slider to view the number of bits borrowed.
Usable hosts per subnet: 218 – 2 = 262,144 – 2 = 262,142
Subnet = 8,192
Hosts per Subnet = 2,048
Subnet = 128
Hosts per Subnet = 131,072
Usable hosts per subnet: 26 – 2 = 64 – 2 = 62
Subnet = 4
Hosts per Subnet = 4,194,304
Subnet = 524,288
Hosts per Subnet = 32
Usable hosts per subnet: 217 – 2 = 131,072 – 2 = 131,070
Usable hosts per subnet: 25 – 2 = 32 – 2 = 30
Usable hosts per subnet: 216 – 2 = 65,536 – 2 = 65,534
Subnet = 256
Hosts per Subnet = 65,536
Subnet = 16,384
Hosts per Subnet = 1,024
Usable hosts per subnet: 24 – 2 = 16 – 2 = 14
Subnet = 8
Hosts per Subnet = 2,097,152
Subnet = 1,048,576
Hosts per Subnet = 16
Usable hosts per subnet: 215 – 2 = 32,768 – 2 = 32,766
Usable hosts per subnet: 23 – 2 = 8 – 2 = 6
Usable hosts per subnet: 214 – 2 = 16,384 – 2 = 16,382
Subnet = 512
Hosts per Subnet = 32,768
Subnet = 32,768
Hosts per Subnet = 512
Subnet = 16
Hosts per Subnet = 1,048,576
Usable hosts per subnet: 223 – 2 = 8,388,608 – 2 = 8,388,606
The usable hosts per subnet equals 2n where n equals the number of zeros, minus the two unusable hosts.
Subnet = 2,097,152
Hosts per Subnet = 8
Usable Hosts per Subnet
Usable hosts per subnet: 213 – 2 = 8,192 – 2 = 8,190
Usable hosts per subnet: 224 – 2 = 16,777,216 – 2 = 16,777,214
Usable hosts per subnet: 212 – 2 = 4,096 – 2 = 4,094
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Subnet = 1,024
Hosts per Subnet = 16,384
Subnet = 65,536
Hosts per Subnet = 256
Usable hosts per subnet: 22 – 2 = 4 – 2 = 2
Subnet = 32
Hosts per Subnet = 524,288
Subnet = 4,194,304
Hosts per Subnet = 4
Usable hosts per subnet: 211 – 2 = 2,048 – 2 = 2,046
Use the slider below to see the effect of borrowing bits from the subnet mask. Slide to the right to increase the number of bits borrowed, slide it to the left to decrease the bits borrowed.
Usable hosts per subnet: 222 – 2 = 4,194,304 – 2 = 4,194,302
Usable hosts per subnet: 20 – 2 = 0 – 2 = NA
Usable hosts per subnet: 210 – 2 = 1,024 – 2 = 1,022
Subnet = 2
Hosts per Subnet = 8,388,608
Subnet = 2,048
Hosts per Subnet = 8,192
Usable hosts per subnet: 221 – 2 = 2,097,152 – 2 = 2,097,150
Subnet = 131,072
Hosts per Subnet = 128
Usable hosts per subnet: 21 – 2 = 2 – 2 = NA
Subnet = 8,388,608
Hosts per Subnet = NA
Usable hosts per subnet: 29 – 2 = 512 – 2 = 510
Usable hosts per subnet: 220 – 2 = 1,048,576 – 2 = 1,048,574
Subnet = 64
Hosts per Subnet = 262,144
Usable hosts per subnet: 28 – 2 = 256 – 2 = 254
Subnet = 0
Hosts per Subnet = 16,777,216
Subnet = 262,144
Hosts per Subnet = 64
Subnet = 4,096
Hosts per Subnet = 4,096
Usable hosts per subnet: 219 – 2 = 524,288 – 2 = 524,286
Subnet = 16,777,216
Hosts per Subnet = NA
Usable hosts per subnet: 27 – 2 = 128 – 2 = 126
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Test Your Knowledge 1 of 8
Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
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Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
Test Your Knowledge 2 of 8
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Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
Test Your Knowledge 3 of 8
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Test Your Knowledge 4 of 8
Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
Test Your Knowledge 5 of 8
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Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
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Test Your Knowledge 6 of 8
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Bits Borrowed Slider
Test Your Knowledge 7 of 8
Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
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Test Your Knowledge 8 of 8
Borrowed Bit Count =
Mask = . . .
Number of Subnets =
Usable Hosts per Subnet =
Go to http://www.t1shopper.com/tools/calculate/ip-subnet/ to create the full table.
4th to last Subnet
Network: 15.255.252.0
Broadcast:
Host Addresses:
–
[x] close
2nd Subnet
Network: 15.0.1.0
Broadcast: 15.0.1.255
Host Addresses:
–
Sample Subnet Class A
Network Address: 15.0.0.0
Bits Borrowed:
Number of Subnets:
65536
Usable Hosts per Subnet: 254
New Subnet Mask:
255.
Magic Number: 0.0.1.0
3rd to last Subnet
Network: 15.255.253.0
Broadcast:
Host Addresses:
–
?? Subnet
Network: 15.0.64.0
Broadcast:
Host Addresses:
–
Last Subnet
Network:
Broadcast: 15.255.255.255
Host Addresses:
–
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1st Subnet
Network: 15.0.0.0
Broadcast: 15.0.0.255
Host Addresses:
–
4th Subnet
Network:
Broadcast:
Host Addresses:
15.0.3.1 – 15.0.3.254
3rd Subnet
Network:
Broadcast:
Host Addresses:
15.0.2.1 – 15.0.2.254
Subnet Challenge 1
2nd to last Subnet
Network:
Broadcast: 15.255.254.255
Host Addresses:
–
View Magic Number Table
3rd to last Subnet
Network: 28.255.208.0
Broadcast:
Host Addresses:
–
Last Subnet
Network:
Broadcast: 28.255.255.255
Host Addresses:
–
?? Subnet
Network: 28.15.208.0
Broadcast:
Host Addresses:
–
1st Subnet
Network: 28.0.0.0
Broadcast: 28.0.15.255
Host Addresses:
–
3rd Subnet
Network:
Broadcast:
Host Addresses:
28.0.32.1 – 28.0.47.254
2nd Subnet
Network: 28.0.16.0
Broadcast: 28.0.31.255
Host Addresses:
–
4th to last Subnet
Network: 28.255.192.0
Broadcast:
Host Addresses:
–
4th Subnet
Network:
Broadcast:
Host Addresses:
28.0.48.1 – 28.0.63.254
2nd to last Subnet
Network:
Broadcast: 28.255.239.255
Host Addresses:
–
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Subnet Challenge 2
Sample Subnet Class A
Network Address:
28.0.0.0
Bits Borrowed: 12
Number of Subnets:
4096
Usable Hosts per Subnet: 4094
New Subnet Mask:
255.255.240.0
Magic Number:
0.0.16.0 or multiplier
Subnet Challenge 3
1st Subnet
Network: 12.0.0.0
Broadcast: 12.31.255.255
Host Addresses:
–
2nd to last Subnet
Network:
Broadcast:
Host Addresses:12.192.0.1 –
--- Subnet
Network: 15.0.64.0
Broadcast:
Host Addresses:
–
3rd Subnet
Network:
Broadcast:
Host Addresses:
12.64.0.1 – 12.95.255.254
4th to last Subnet
Network: 12.128.0.0
Broadcast:
Host Addresses:
–
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Sample Subnet Class A
Network Address: 12.0.0.0
Bits Borrowed:
Number of Subnets: 8
Usable Hosts per Subnet:
New Subnet Mask:
Magic Number:
0.32.0.0 or multiplier
4th Subnet
Network:
Broadcast:
Host Addresses:
12.96.0.1 – 12.127.255.254
3rd to last Subnet
Network:
Broadcast: 12.191.255.255
Host Addresses:
–
2nd Subnet
Network: 12.32.0.0
Broadcast: 12.63.255.255
Host Addresses:
–
Last Subnet
Network:
Broadcast:
Host Addresses: 12.224.0.1 –
3rd to last Subnet
Network:
Broadcast: 55.255.247.255
Host Addresses:
–
Last Subnet
Network:
55.255.252.0
Broadcast:
Host Addresses:
Subnet Challenge 4
?? Subnet
Network:
55.3.248.0
Broadcast:
Host Addresses:
–
1st Subnet
Network:
Broadcast: 55.0.3.255
Host Addresses:
55.0.0.1 -
55.0.3.254
4th to last Subnet
Network:
Broadcast: 55.255.243.255
Host Addresses:
–
3rd Subnet
Network: 55.0.8.0
Broadcast:
Host Addresses:
55.0.8.1 -
55.0.11.254
4th Subnet
Network:55.0.12.0
Broadcast:
Host Addresses:
55.0.12.1 -
55.0.15.254
2nd Subnet
Network:
Broadcast: 55.0.7.255
Host Addresses:
55.0.4.1 -
55.0.7.254
Sample Subnet Class A
Network Address: 55.0.0.0
Bits Borrowed:
Number of Subnets: 16,384
Usable Hosts per Subnet:
New Subnet Mask:
255.255.252.0
Magic Number: 0.0.4.0 or multiplier
2nd to last Subnet
Network:
55.255.248.0
Broadcast:
Host Addresses:
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Subnet Challenge 5
3rd to last Subnet
Network:
Broadcast:
Host Addresses:
50.255.160.1 –
Last Subnet
Network:
Broadcast:
50.255.255.255
Host Addresses:–
1st Subnet
Network: 50.0.0.0
Broadcast: 50.0.31.255
Host Addresses:
–
3rd Subnet
Network: 50.0.64.0
Broadcast:
Host Addresses:
–
4th to last Subnet
Network:
50.255.128.0
Broadcast:
Host Addresses:
–
4th Subnet
Network:
Broadcast:
Host Addresses:
50.0.96.1 – 50.0.127.254
2nd Subnet
Network: 50.0.32.0
Broadcast: 50.0.63.255
Host Addresses:
–
Sample Subnet Class A
Network Address: 50.0.0.0
Bits Borrowed: 11
Number of Subnets:
Usable Hosts per Subnet:
New Subnet Mask:
255.
Magic Number:
0.0.32.0 or multiplier
2nd to last Subnet
Network:
Broadcast:
50.255.223.255
Host Addresses:–
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?? Subnet
Network:
50.100.128.0
Broadcast:
Host Addresses:
–
4th Subnet
Network: 12.0.0.24
Broadcast: 12.0.0.31
Host Addresses:
–
Subnet Challenge 6
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3rd to last Subnet
Network:
12.255.255.232
Broadcast:
Host Addresses:
–
2nd to last Subnet
Network:
Broadcast:
Host Addresses:
12.255.255.241 –
?? Subnet
Network:
12.255.248.112
Broadcast:
Host Addresses:
–
Last Subnet
Network:
Broadcast:
12.255.255.255
Host Addresses:–
3rd Subnet
Network: 12.0.0.16
Broadcast: 12.0.0.23
Host Addresses:
–
Sample Subnet Class A
Network Address: 12.0.0.0
Bits Borrowed: 21
Number of Subnets:
Usable Hosts per Subnet:
New Subnet Mask:
Magic Number:
0.0.0.8 or multiplier
1st Subnet
Network:
Broadcast:
Host Addresses:
12.0.0.1 – 12.0.0.6
4th to last Subnet
Network:
12.255.255.224
Broadcast:
Host Addresses:
–
2nd Subnet
Network:
Broadcast:
Host Addresses:
12.0.0.9 – 12.0.0.14
4th to last Subnet
Network:
Broadcast:
75.255.255.159
Host Addresses:–
Sample Subnet Class A
Network Address: 75.0.0.0
Bits Borrowed: 19
Number of Subnets:
Usable Hosts per Subnet:
New Subnet Mask:
Magic Number:
0.0.0.32 or multiplier
2nd Subnet
Network: 75.0.0.32
Broadcast: 75.0.0.63
Host Addresses:
–
Subnet Challenge 7
3rd Subnet
Network:
Broadcast:
Host Addresses:
75.0.0.65 – 75.0.0.94
Last Subnet
Network:
Broadcast:
Host Addresses:–75.255.255.254
1st Subnet
Network: 75.0.0.0
Broadcast: 75.0.0.31
Host Addresses:
–
4th Subnet
Network:
Broadcast:
Host Addresses:
75.0.0.97 – 75.0.0.126
3rd to last Subnet
Network:
Broadcast:
Host Addresses:
75.255.255.161 –
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?? Subnet
Network:
75.255.239.128
Broadcast:
Host Addresses:
–
2nd to last Subnet
Network:
Broadcast:
75.255.255.223
Host Addresses:
– 75.255.255.222
Last Subnet
Network:
Broadcast:
Host Addresses:– 17.255.255.254
4th Subnet
Network:
Broadcast:
Host Addresses:
17.0.6.1 – 17.0.7.254
Subnet Challenge 8
2nd Subnet
Network: 17.0.2.0
Broadcast: 17.0.3.255
Host Addresses:
–
4th to last Subnet
Network: 17.255.248.0
Broadcast:
Host Addresses:–
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1st Subnet
Network: 17.0.0.0
Broadcast: 17.0.1.255
Host Addresses:
–
?? Subnet
Network: 17.255.14.0
Broadcast:
Host Addresses:
–
3rd to last Subnet
Network: 17.255.250.0
Broadcast:
Host Addresses:–
2nd to last Subnet
Network:
Broadcast:
Host Addresses:
17.255.252.1 –
Sample Subnet Class A
Network Address: 17.0.0.0
Bits Borrowed:
Number of Subnets:
Usable Hosts per Subnet: 510
New Subnet Mask:
Magic Number:
0.0.2.0 or multiplier
3rd Subnet
Network:
Broadcast:
Host Addresses:
17.0.4.1 – 17.0.5.254
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Network Increment Slider
Move the slider down to see the bits borrowed from the host portion of the address