Subnet Mask

32-bit number created by setting host bits to all 0s & network bits to all 1s and is used to distinguish between a network address and a host address in IP address.

IP Class	Leading Bits	Start Address	End Address	Default Subnet Mask	No. of Networks	No. of hosts per network	Use
A	0xxx	1.0.0.0	126.0.0.0	255.0.0.0	126	16,777,214	Large organizations
B	10xx	128.0.0.0	191.255.0.0	255.255.0.0	16,384	65,534	Medium-sized organizations
C	110x	192.0.0.0	223.255.255.0	255.255.255.0	2,097,152	254	Small organizations
D	1110	224.0.0.0	239.255.255.255	Not applicable	N/A	N/A	Multicast groups
E	1111	240.0.0.0	255.255.255.254	Not applicable	N/A	N/A	Experimental; used for research

Default Subnet Mask

IP Class	Leading Bits	Network Bits	Host Bits	Slash	Dotted Decimal
A	0	1111111	00000000 00000000 00000000	/8	255.0.0.0
B	10	111111 11111111	00000000 00000000	/16	255.255.0.0
C	110	11111 11111111 11111111	00000000	/24	255.255.255.0
D	1110	Multicast Group ID	Multicast Group ID	-	-
E	1111	-	-	-	-

Despite of having 0 in the leading bits how come 011111111 = 255?

The subnet mask reflects the division between the network and host portions of the address, not the specific values of the bits themselves. So even though the first bit in a Class A address is 0, the subnet mask for Class A is still 255.0.0.0 because all 8 bits of the first octet are used for the network portion.

<aside> 💡 No Subnetting is done -> 10.0.0.0/8 128.0.0.0/16 192.0.0.0/24 (By Default) Subnetting is done -> 10.0.0.0/11 128.0.0.0/12 192.0.0.0/15

</aside>

Subnetting

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
Subnet 255.x.0.0 (Class A)
CIDR	/9	/10	/11	/12	/13	/14	/15	/16
Hosts	8,388,608	4,194,304	2,097,152	1,048,576	524,288	262,144	131,072	65,536
Subnet 255.255.x.0 (Class B)
CIDR	/17	/18	/19	/20	/21	/22	/23	/24
Hosts	32,768	16,384	8,192	4,096	2,048	1,024	512	256
Subnet 255.255.255.x (Class C)
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

Subnetting Examples

<aside> 💡 Hosts double with each increment of a CIDR.

</aside>

Always subtract 2 from host total:

Network ID - First Address
Broadcast - Last Address
The first and last bits are not always network and broadcast IDs.

CIDR to IPv4 Address Range Utility Tool | IPAddressGuide

Private IP Addresses

Network Class	Start Address	End Address	Network Mask	No. of Networks	No. of hosts per network
Class A	10.0.0.0	10.255.255.255	255.0.0.0	126	16,646,144
Class B	172.16.0.0	172.31.255.255	255.255.0.0	16,383	65,024
Class C	192.168.0.0	192.168.255.255	255.255.255.0	2,097,151	254
Lookback (Localhost)	127.0.0.0	127.0.0.7	255.255.255.0	-	-