Finding the subnet 1
147.50.108.10/19
255.255.224.0
- Copy IP address and mast into table
- Draw rectangle around interesting octet. Interesting octet is octet which subnet mask value is not 255 or 0.
- Copy IP addresses from octets where mask is 255 to the Subnet number row and where mask is zero, type also 0.
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Calculation SPECIAL number:
SPECIAL= 256 – subnet_mask_interesting_octet
SPECIAL = 256-224
SPECIAL= 32
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Multiplies SPECIAL number:
0, 32, 64, 96, 128, 160, 192, 224, 256
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Compare number IP address number (108) from interesting field with the multiplied SPECIAL numbers. You can see that 108 is between 96 and 128. So 96 is subnet number.
Finding the subnet 2
10.84.88.25/21
255.255.248.0
- Copy IP address and mast into table
- Draw rectangle around interesting octet. Interesting octet is octet which value is not 255 or 0.
- Copy IP addresses from octets where mask is 255 to the Subnet number row and where mask is zero, type also 0.
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Calculation SPECIAL number:
SPECIAL= 256 – subnet_mask_interesting_octet
SPECIAL = 256-248
SPECIAL= 8
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Multiplies SPECIAL number:
0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96 ….256
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Compare number IP address number (88) from interesting field with the multiplied SPECIAL numbers. You can see that 88 is subnet number
Finding the subnet 3
192.168.1.148/28
255.255.255.240
- Copy IP address and mast into table
- Draw rectangle around interesting octet. Interesting octet is octet which value is not 255 or 0.
- Copy IP addresses from octets where mask is 255 to the Subnet number row and where mask is zero, type also 0.
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Calculation SPECIAL number:
SPECIAL= 256 – subnet_mask_interesting_octet
SPECIAL = 256-240
SPECIAL= 16
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Multiplies SPECIAL number:
0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160... -
Compare number IP address number (148) from interesting field with the multiplied SPECIAL numbers. You can see that 148 is between 144 and 160. Subnet number is 144.
Source:CCENT/CCNA ICND1 Official Exam Certification Guide, Second Edition [Wendell Odom]