Sunday, June 10, 2018

Introduction to Subnetting

Introduction to Subnetting

A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.

The process of subnetting involves the separation of the network and subnet portion of an address from the host identifier. This is performed by a bitwise AND operation between the IP address and the (sub)network mask. The result yields the network address or prefix, and the remainder is the host identifier.


Fig 1.1- Sample Class C Subnetting

Determining the network prefix

An IPv4 network mask consists of 32 bits, a sequence of ones (1) followed by a block of 0s. The trailing block of zeros (0) designates that part as being the host identifier.
The following example shows the separation of the network prefix and the host identifier from an address (192.168.5.130) and its associated /24 network mask (255.255.255.0). The operation is visualized in a table using binary address formats.

Determining the network prefix

An IPv4 network mask consists of 32 bits, a sequence of ones (1) followed by a block of 0s. The trailing block of zeros (0) designates that part as being the host identifier.
The following example shows the separation of the network prefix and the host identifier from an address (192.168.5.130) and its associated /24 network mask (255.255.255.0). The operation is visualized in a table using binary address formats.
Binary formDot-decimal notation
IP address11000000.10101000.00000101.10000010192.168.5.130
Subnet mask11111111.11111111.11111111.00000000255.255.255.0
Network prefix11000000.10101000.00000101.00000000192.168.5.0
Host part00000000.00000000.00000000.100000100.0.0.130
The mathematical operation for calculating the network prefix is the bitwise AND of IP address and subnet mask. The result of the operation yields the network prefix 192.168.5.0 and the host number 130.

Subnetting

Subnetting is the process of designating some high-order bits from the host part and grouping them with the network mask to form the subnet mask. This divides a network into smaller subnets. The following diagram modifies the example by moving 2 bits from the host part to the subnet mask to form four smaller subnets one quarter the previous size:
Binary formDot-decimal notation
IP address11000000.10101000.00000101.10000010192.168.5.130
Subnet mask11111111.11111111.11111111.11000000255.255.255.192
Network prefix11000000.10101000.00000101.10000000192.168.5.128
Host part00000000.00000000.00000000.000000100.0.0.2

Prefix sizeNetwork maskAvailable
subnets
Usable hosts
per subnet
Total
usable hosts
/24255.255.255.01254254
/25255.255.255.1282126252
/26255.255.255.192462248
/27255.255.255.224830240
/28255.255.255.2401614224
/29255.255.255.248326192
/30255.255.255.252642128
/31255.255.255.254128*256

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