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Practice Subnetting

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 10.125.221.5/25?

Click for a new random question.

Reveal The Answer

Reveal The Work

1) Convert the CIDR bit number to a decimal subnet mask ('Useful Graph').
25	=>	255.255.255.128
2) Convert the IP address and subnet mask to binary.
Host IP		10.125.221.5	=>	00001010.01111101.11011101.00000101
Subnet Mask		255.255.255.128	=>	11111111.11111111.11111111.10000000
3) Logically AND the binary IP and mask together.
Host IP		10.125.221.5		00001010.01111101.11011101.00000101
Subnet Mask		255.255.255.128		11111111.11111111.11111111.10000000
AND				00001010.01111101.11011101.00000000
4) Convert the AND result to decimal. This is the subnet IP address.
Host IP		10.125.221.5		00001010.01111101.11011101.00000101
Subnet Mask		255.255.255.128		11111111.11111111.11111111.10000000
AND		10.125.221.0	<=	00001010.01111101.11011101.00000000
5) Inverse the binary digits of the subnet mask (from step 2) to create the inverse mask.
Inverse Mask				00000000.00000000.00000000.01111111
6) Logically OR the binary IP and inverse mask together.
Host IP		10.125.221.5		00001010.01111101.11011101.00000101
Inverse Mask				00000000.00000000.00000000.01111111
OR				00001010.01111101.11011101.01111111
7) Convert the OR result to decimal. This is the broadcast IP address.
Host IP		10.125.221.5		00001010.01111101.11011101.00000101
Inverse Mask				00000000.00000000.00000000.01111111
OR		10.125.221.127	<=	00001010.01111101.11011101.01111111

Practice Subnetting

Subnetting Notes

Boolean Logic

Useful Graph

Subnetting Question

Reveal The Answer

Reveal The Work

1) Convert the CIDR bit number to a decimal subnet mask ('Useful Graph').

2) Convert the IP address and subnet mask to binary.

3) Logically AND the binary IP and mask together.

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

5) Inverse the binary digits of the subnet mask (from step 2) to create the inverse mask.

6) Logically OR the binary IP and inverse mask together.

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