What Is Subnetting?

What Is Subnetting?

Subnetting is a method for getting the most out of the limited 32-bit IPv4 addressing space and reducing the size of the routing tables in a large internetwork. With any address class, subnetting provides a means of allocating a part of the host address space to network addresses, which lets you have more networks. The part of the host address space allocated to new network addresses is known as the subnet number.
In addition to making more efficient use of the IPv4 address space, subnetting has several administrative benefits. Routing can become very complicated as the number of networks grows. A small organization, for example, might give each local network a class C number. As the organization grows, administering a number of different network numbers could become complicated. A better idea is to allocate a few class B network numbers to each major division in an organization. For instance, you could allocate one to Engineering, one to Operations, and so on. Then, you could divide each class B network into additional networks, using the additional network numbers gained by subnetting. This can also reduce the amount of routing information that must be communicated among routers.

Class	Address	# of Hosts	Netmask (Binary)	Netmask (Decimal)
CIDR	/4	240,435,456	11110000 00000000 00000000 00000000	240.0.0.0
CIDR	/5	134,217,728	11111000 00000000 00000000 00000000	248.0.0.0
CIDR	/6	67,108,864	11111100 00000000 00000000 00000000	252.0.0.0
CIDR	/7	33,554,432	11111110 00000000 00000000 00000000	254.0.0.0
A	/8	16,777,216	11111111 00000000 00000000 00000000	255.0.0.0
CIDR	/9	8,388,608	11111111 10000000 00000000 00000000	255.128.0.0
CIDR	/10	4,194,304	11111111 11000000 00000000 00000000	255.192.0.0
CIDR	/11	2,097,152	11111111 11100000 00000000 00000000	255.224.0.0
CIDR	/12	1,048,576	11111111 11110000 00000000 00000000	255.240.0.0
CIDR	/13	524,288	11111111 11111000 00000000 00000000	255.248.0.0
CIDR	/14	262,144	11111111 11111100 00000000 00000000	255.252.0.0
CIDR	/15	131,072	11111111 11111110 00000000 00000000	255.254.0.0
B	/16	65,534	11111111 11111111 00000000 00000000	255.255.0.0
CIDR	/17	32,768	11111111 11111111 10000000 00000000	255.255.128.0
CIDR	/18	16,384	11111111 11111111 11000000 00000000	255.255.192.0
CIDR	/19	8,192	11111111 11111111 11100000 00000000	255.255.224.0
CIDR	/20	4,096	11111111 11111111 11110000 00000000	255.255.240.0
CIDR	/21	2,048	11111111 11111111 11111000 00000000	255.255.248.0
CIDR	/22	1,024	11111111 11111111 11111100 00000000	255.255.252.0
CIDR	/23	512	11111111 11111111 11111110 00000000	255.255.254.0
C	/24	256	11111111 11111111 11111111 00000000	255.255.255.0
CIDR	/25	128	11111111 11111111 11111111 10000000	255.255.255.128
CIDR	/26	64	11111111 11111111 11111111 11000000	255.255.255.192
CIDR	/27	32	11111111 11111111 11111111 11100000	255.255.255.224
CIDR	/28	16	11111111 11111111 11111111 11110000	255.255.255.240
CIDR	/29	8	11111111 11111111 11111111 11111000	255.255.255.248
CIDR	/30	4	11111111 11111111 11111111 11111100	255.255.255.252