Question 1

How do I find the network address of a subnet?

Accepted Answer

Perform a bitwise AND between the IP address and the subnet mask. For example, 192.168.1.130 AND 255.255.255.128 (/25) gives 192.168.1.128. The network address is always the lowest address in the block.

Question 2

How do I find the broadcast address?

Accepted Answer

Set all host bits to 1. For a /25 starting at 192.168.1.128, there are 7 host bits, so the broadcast is 192.168.1.128 + 127 = 192.168.1.255. Alternatively: network address OR wildcard mask.

Question 3

How many usable hosts are in a subnet?

Accepted Answer

Usable hosts = 2^(host bits) − 2. The −2 removes the network address and broadcast address. A /24 has 8 host bits, so 2^8 − 2 = 254 usable hosts. A /30 has 2 host bits: 2^2 − 2 = 2 usable hosts.

Prefix	Subnet Mask	Magic No.	Block Size	Usable Hosts	Wildcard
/16	255.255.0.0	—	65,536	65,534	0.0.255.255
/17	255.255.128.0	128	32,768	32,766	0.0.127.255
/18	255.255.192.0	64	16,384	16,382	0.0.63.255
/19	255.255.224.0	32	8,192	8,190	0.0.31.255
/20	255.255.240.0	16	4,096	4,094	0.0.15.255
/21	255.255.248.0	8	2,048	2,046	0.0.7.255
/22	255.255.252.0	4	1,024	1,022	0.0.3.255
/23	255.255.254.0	2	512	510	0.0.1.255
/24	255.255.255.0	—	256	254	0.0.0.255
/25	255.255.255.128	128	128	126	0.0.0.127
/26	255.255.255.192	64	64	62	0.0.0.63
/27	255.255.255.224	32	32	30	0.0.0.31
/28	255.255.255.240	16	16	14	0.0.0.15
/29	255.255.255.248	8	8	6	0.0.0.7
/30	255.255.255.252	4	4	2	0.0.0.3

Subnetting Practice Quiz

Quick Reference — Magic Numbers

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