Subnet – How to find number of subnets

subnet

How to find the total number of subnets available for one subnet mask?.

For example, take this subnet mask:

255.255.255.224 and IP address as 192.168.5.139, book shows that there are 6 subnet for the given subnet mask, ranging from 192.168.5.32 to 192.168.5.192 , how 6 ?

I tried to solve it like this 2^3 = 8 and 8-2 = 6 But I thought that we should subtract 2 only when we find number of hosts (because of network ID and Broadcast).

Why do we subtract 2 for subnets?

Best Answer

If a network address is subnetted, the first subnet obtained after subnetting the network address is called subnet zero and the last subnet obtained is called the all-ones subnet.

In your example:

Network: 192.168.5.0

Subnet Mask: 255.255.255.224

Subnets:

Subnet            Broadcast
192.168.5.0      192.168.0.31   <-- subnet zero
 
192.168.5.32     192.168.0.63

192.168.5.64     192.168.0.95

192.168.5.96     192.168.0.127

192.168.5.128    192.168.0.159

192.168.5.160    192.168.0.191

192.168.5.192    192.168.0.223

192.168.5.224    192.168.0.255    <-- all ones

Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing.

According to RFC 950 from 1985:

"It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets."

Remember that this is a 1985 recommendation. More than 30 years have passed from then.

Using subnet zero for addressing is discouraged because of the confusion inherent in having a network (192.168.5.0) and a subnet (192.169.5.0) with indistinguishable addresses, and the all-ones subnet for the confusion inherent in having a network and a subnet with identical broadcast addresses (192.168.5.255).

This recommendation was overruled in 1995 by RFC-1878 and RFC-1812. So, nowadays we can use all the subnets and the devices' hardware /software support it.