How to find number of borrowed bits in subnetting

Question

I am an Electrical Engineer learning about computer Networks, So this question might be childish and easy for most of you. Please answer it briefly. I am performing subnetting where I have this address 192.10.10.0. In this worksheet, I can understand all the data except a number of borrowed bits which are 4. Please tell me How and from where this 4 came from? I have downloaded different worksheets and docs but was not able to understand this.

Answer 1

The borrowed bits are the host bits in the default subnet mask that are used for network in the custom subnet mask.

In your example:

Last byte of the default subnet mask:

Binary      Decimal
00000000 -> 0

Last byte of the custom subnet mask:

Binary      Decimal
11110000 -> 240

The first four (4) bits go from host to network, they are borrowed bits.

It can be understood easily using CIDR notation:

Netmask           CIDR Notation
255.255.255.255   32

Netmask           CIDR Notation
255.255.255.0     24

Netmask           CIDR Notation
255.255.255.240   28