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Practice Subnetting

Subnetting Help

Boolean Logic

0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1

0 OR 0 = 0
0 OR 1 = 1
1 OR 0 = 1
1 OR 1 = 1

Useful Graph

This graph helps me reduce the number of errors I make while answering subnettting questions on exams. I typically draw it as I sit down to an exam and then use it as a reference while checking my work before submitting an answer.

Find the column with the corresponding prefix bits as the practice question you are working on and then pay attention to how that correlates to the "Magic number" and "Subnet mask" rows.

Magic number	128	64	32	16	8	4	2	1
Prefix bits	1	2	3	4	5	6	7	8
Prefix bits	9	10	11	12	13	14	15	16
Prefix bits	17	18	19	20	21	22	23	24
Prefix bits	25	26	27	28	29	30	31	32
Subnet mask	128	192	224	240	248	252	254	255

Subnetting Question

What are the subnet and broadcast IPs for host 10.122.115.254/13?

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Reveal The Answer

Reveal The Work

Convert the CIDR bit number to a subnet mask.
13	255.248.0.0
Convert the IP address and subnet mask to binary.
Host IP	10.122.115.254	00001010.01111010.01110011.11111110
Subnet Mask	255.248.0.0	11111111.11111000.00000000.00000000
Logically AND the binary IP and mask together.
Host IP	10.122.115.254	00001010.01111010.01110011.11111110
Subnet Mask	255.248.0.0	11111111.11111000.00000000.00000000
AND		00001010.01111000.00000000.00000000
Convert the AND result to decimal. This is the subnet IP address.
Host IP	10.122.115.254	00001010.01111010.01110011.11111110
Subnet Mask	255.248.0.0	11111111.11111000.00000000.00000000
AND	10.120.0.0	00001010.01111000.00000000.00000000
Inverse the binary digits of the subnet mask.
Host IP	10.122.115.254	00001010.01111010.01110011.11111110
Subnet Mask	255.248.0.0	11111111.11111000.00000000.00000000
Inverse Mask		00000000.00000111.11111111.11111111
Logically OR the binary IP and inverse mask together.
Host IP	10.122.115.254	00001010.01111010.01110011.11111110
Inverse Mask		00000000.00000111.11111111.11111111
OR		00001010.01111111.11111111.11111111
Convert the OR result to decimal. This is the broadcast IP address.
Host IP	10.122.115.254	00001010.01111010.01110011.11111110
Inverse Mask		00000000.00000111.11111111.11111111
OR	10.127.255.255	00001010.01111111.11111111.11111111

Subnetting Help

Boolean Logic

Useful Graph

Subnetting Question

Reveal The Answer

Reveal The Work

Convert the CIDR bit number to a subnet mask.

Convert the IP address and subnet mask to binary.

Logically AND the binary IP and mask together.

Convert the AND result to decimal. This is the subnet IP address.

Inverse the binary digits of the subnet mask.

Logically OR the binary IP and inverse mask together.

Convert the OR result to decimal. This is the broadcast IP address.