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 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? enter image description here I have downloaded different worksheets and docs but was not able to understand this.

  • See the excellent answer to this question. – Ron Maupin Jun 29 '17 at 13:49
  • Could you explain me where is borrow part in the question you are linking to OR you are just a fan of marking duplicates? – Electrical Engineering Jun 29 '17 at 16:16
  • There is a section called Calculating subnets that explains this. Also, understand that network classes are dead, killed in 1993 by RFCs 1518 and 1519, which defined CIDR (Classless Inter-Domain Routing). Modern networking does not use network classes. Please let them rest in peace. Also, all "education, certification, or homework" questions are explicitly off-topic here. – Ron Maupin Jun 29 '17 at 16:20

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   32

Netmask           CIDR Notation     24

Netmask           CIDR Notation   28
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