What is Subnetting?|Hacker's Adda

What is Subnetting?

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Subnetting is the process of breaking down an IP network into smaller sub-networks called "subnets." Each subnet is a non-physical description (or ID) for a physical sub-network (usually a switched network of host containing a single router in a multi-router network).

Subnet Mask Notation

There are two forms of subnet notation, standard notation and CIDR (Classless Internet Domain Routing) notation. Both versions of notation use a base address (or network address) to define the starting point of the network, such as 192.168.1.0. This means that the network begins at 192.168.1.0 and the first possible host IP address on this subnet would be 192.168.1.1.

In standard subnet mask notation, a four octet numeric value is used as with the base address, for example 255.255.255.0. The standard mask can be calculated by creating four binary values for each octet, and assigning the binary digit of .1. to the network potion, and assigning the binary digit of .0. to the network potion. In the example above this value would be 11111111.11111111.11111111.00000000. In combination with the base address you have a subnet definition, in this case the subnet in standard notation would be 192.168.1.0 255.255.255.0.

In CIDR notation, the number of 1.s in the binary version of the mask are counted from the left, and that number is appended to the end of the base address following a slash (/). In the example here the subnet would be listed in CIDR notation as 192.168.1.0/24.

When is Subnetting Used?

Subnets are created to limit the scope of broadcast traffic, to apply network security measures, to separate network segments by function, and/or to assist in resolving network congestion problems..,

A subnet is usually composed of a network router, a switch or hub, and at least one host

How can I Compute the Maximum Number of Hosts for a Subnet Mask?

To compute the maximum number of hosts for a subnet mask, take two and raise it to the amount of bits allocated to the subnet (count the number of 0.s in the subnet mask binary value) and subtract two. You must subtract two from the resulting value because the first value in the IP address range (all 0s) is reserved for the network address, and the last value in the IP address range (all 1s) is reserved for the network broadcast address. For example, DSL networks commonly use 8 bits for their subnets. The amount of allowable hosts for such a DSL network could be computed by the following formula: max hosts = (2^8)-2 = 254 hosts.

As you subnet networks, the number of bits represented by the subnet mask will decrease. You decrease the octets in order starting from the rightmost value and proceed left as you reach a zero value. Mask values decrease by a power of two each time you split a network into more subnets. Values are 255, 254*, 252, 248, 240, 224, 224, 192, 128. Each decrease indicates that an additional bit has been allocated. After 128, the next bit allocated will reduce the fourth octet to 0; and the third octet will follow same 8-number progression.

For instance, a subnet mask dotted decimal number of 255.255.255.255 indicates that no bits have been allocated and that the maximum number of hosts is 1 (0^1=1). The subnet mask 255.255.255.128 indicates that the maximum number of hosts is 128. And the subnet mask 255.255.128.0 indicates that the maximum number of hosts is 32,786.

* 254 is not a valid number for the fourth octet because no addresses are available for hosts. i.e. (2^1)-2 = 0.