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Bronze

number of vlaid subnets

Hi every body!

Let say we have class C 192.192.192.0 network with subnet mask 255.255.255.192

The total number of subnets are:

192.192.192.0

192.192.192.64

192.192.192.128

192.192.192.192

According to cisco book , the valid subnnets are = total subnets-2

The subnet "192.192.192.0 " and " 192.192.192.192" are not considered valid subnets . My question is why?

Thanks a lot!

2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

Re: number of vlaid subnets

Sarah,

Here's a paper that explains why usage of the subnet zero and all one's subnet was discouraged by RFC950.

Subnet zero and all one's are now widely used.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Silver

Re: number of vlaid subnets

Sarah

To be bit less subtle that Harold, the book you reference is presenting a very old (and out moded) view of the principles of subnetting. In modern practice neither of the subnets is not permitted. (you might however, be careful on certification tests, some of which may still reflect the old practice).

HTH

Rick

3 REPLIES
Hall of Fame Super Gold

Re: number of vlaid subnets

IP Addresses 192.192.192.0 & 192.192.192.192 are not valid IP ADDRESSES because they are subnet addresses.

I think this is one of those trick questions again.

Cisco Employee

Re: number of vlaid subnets

Sarah,

Here's a paper that explains why usage of the subnet zero and all one's subnet was discouraged by RFC950.

Subnet zero and all one's are now widely used.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Silver

Re: number of vlaid subnets

Sarah

To be bit less subtle that Harold, the book you reference is presenting a very old (and out moded) view of the principles of subnetting. In modern practice neither of the subnets is not permitted. (you might however, be careful on certification tests, some of which may still reflect the old practice).

HTH

Rick

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