Subnetting

Subnetting Sheet

• Hosts double each increment of a CIDR

• Always subtract 2 from host total

• Network ID: First Address

• Broadcast: Last Address

			SUBNET	x.0.0.0
CIDR	/1	/2	/3	/4	/5	/6	/7	/8
Hosts	2,147,483,648	1,073,741,824	536,870,912	268,435,456	134,217,728	67,108,864	33,554,432	16,777,216
Class A			SUBNET	255.x.0.0
CIDR	/9	/10	/11	/12	/13	/14	/15	/16
Hosts	8,388,608	4,194,304	4,194,304	1,048,576	524,288	262,144	131,072	65,536
Class B			SUBNET	255.255.x.0
CIDR	/17	/18	/19	/20	/21	/22	/23	/24
Hosts	32,768	16,384	8,192	4,096	2,048	1,024	512	256
Class C			SUBNET	255.255.255.x
CIDR	/25	/26	/27	/28	/29	/30	/31	/32
Hosts	128	64	32	16	8	4	2	1
Subnet Mask (Replace X)	128	192	224	240	248	252	254	255

Cheat Sheet Examples

	Subnet	Hosts	Network	Broadcast
192.168.1.0/24	255.255.255.0	254	192.168.1.0	192.168.1.255
192.168.1.16/28	255.255.255.240	14	192.168.1.16	192.168.1.31
192.168.0.0/22	255.255.252.0	1022	192.168.0.0	192.168.3.255

Bits and Octets

First Octet

128	64	32	16	8	4	2	1
1	1	1	1	1	1	1	1	Total: 255

Second Octet

128	64	32	16	8	4	2	1
1	1	1	1	1	1	1	1	Total: 255

Third Octet

128	64	32	16	8	4	2	1
1	1	1	1	1	1	1	1	Total: 255

Fourth Octet

128	64	32	16	8	4	2	1
0	0	0	0	0	0	0	0	Total: 0

Result

Resulting CIDR	Hosts (Exponential)	Hosts
/24	2^8	256
(bits on)	2^(bits off)	Resulting hosts

Meaning, a subnet of 255.255.255.0 has 24 bits switched on (3 sets of octets), 8 bits switched off (1 set of octets), which gives it 256 hosts available.

Example: A Class C Private IP Address network of 192.168.86.0/24 has a subnet of 255.255.255.0, and has 256 available hosts. Subtract 2 hosts for the Network ID and Broadcast address, leaves us with 254, So any IP in the range of 192.168.86.1-254 is available to be assigned on that network.