6 Replies Latest reply: Oct 2, 2008 10:09 PM by Lee Phillips RSS

    IP Subnetting Format

    Lee Phillips

       

       

       

       

      Hi,

       

       

       

       

       

      I have recently failed an exam and would greatly appreciate some feedback in regards to the below question. I’m going to structure this question as clearly as I can but please keep in mind that it may not be correct as I didn’t understand it myself, hence the reason for putting my query here.

       

       

       

       

       

      Q: You will need to apply an IP address to the interface based on the below information after enabling RIP version 2:

       

       

       

       

       

      Fa0 IP 192.168.16.185 using the tenth subnet

       

       



       

       

      As it may be quite clear on how to obtain the answer here, I was confused during the exam as I’m use to subnetting questions structured in this format 192.168.16.185 255.255.255.x or 192.168.16.185/2x

       

       

       

       

       

      This may be a question for the Cisco admin/management team to advise me but anyone feel free to share? Furthermore does anyone know where there is a website where I could get examples on how to understand this nature of questions?

       

       

       

       

       

      Kind Regards

       

       

       

       

       

      Lee Phillips

       

       

        • 1. Re: IP Subnetting Format
          Kapila Withanawasam

          Hi Lee,

           

          192.168.16.185 This is a Class C IP address

           

           

          The default mask is 255.255.255.0

           

           

          To create minimum 10 subnets we need to borrow 4 host bits.

           

           

          If you borrow 4 host bits the new subnet mask will be

           

           

          255.255.255.240 and the network increment is 16

           

           

          Therefore the first subnet would be 192.168.16.0

           

           

          The second subnet would be 192.168.16.16

           

           

          The third subnet would be 192.168.16.32

           

           

          The 10th subnet would be 192.168.16.160

           

           

          The first host IP address in 10th subnet would be 192.168.16.161

           

           

          Tha last valid IP address in 10 subnet is 192.168.16.174

           

           

          The broadcast address of the 10 subnet would be 192.168.16.175

           

           

          Your answer should be 192.168.16.161 255.255.255.240

           

           

          If you didnt get this dont worry , I had the same problem.

           

           

          Have a look at this document. This helped me a lot to understand subnetting.

           

           

          There are good websites to practice subnetting.

           

           

          Regards,

           

           

          Kapila

           

           

          • 2. Re: IP Subnetting Format
            Bert-Jan Vosman

             

            ip address 192.168.16.185 /28

             

             

            is on subnet 192.168.16.176

             

             

            first available ip address would be 192.168.16.177

             

             

            • 3. Re: IP Subnetting Format
              cnewby

              Hi Bert-Jan,

               

              Yes, my calculations agree with yours.

               

               

               

              192.168.16.176 is The subnet (12th BTW) First host 192.168.16.177, last host 192.168.16.190. Broadcast 192.168.16.191

               

               

               

              BTW I calculated this manually and checked my work using the "Solarwinds Advance Subnet Calculator" a great free tool. It does NOT generate questions, but sure helps in verifying your answers

               

               

               

              Charles

              • 4. Re: IP Subnetting Format
                Lee Phillips

                Good Evening Kapila,

                 

                I've had time to go over your explanation to my question and am very grateful for you help so far but I'm just a little confused about one thing, please see the below:

                 


                Q: You will need to apply an IP address to the

                interface based on the below information after enabling RIP version 2:

                 

                 

                Fa0 IP 192.168.16.185 using the tenth subnet


                 

                192.168.16.185 - Class C address

                255.255.255.0 - Default Mask

                 

                Borrow 4 host bits which give the new subnet mask: 255.255.255.240 with an increment of 16

                 

                1st Subnet: 192.168.16.0

                2nd Subnet: 192.168.16.16

                3rd Subnet: 192.168.16.32

                4th Subnet: 192.168.16.48

                5th Subnet: 192.168.16.64

                6th Subnet: 192.168.16.80

                7th Subnet: 192.168.16.96

                8th Subnet: 192.168.16.112

                9th Subnet: 192.168.16.128

                10th Subnet: 192.168.16.144

                11th Subnet: 192.168.16.160

                 

                I thought we count "0" as being the first subnet? or am missing something? Just would like to understand this question.

                 

                Shouldn't my answer be: 192.168.16.145 255.255.255.240

                 

                Please can you advise?

                 

                Kind Regards

                 

                Lee Phillips

                • 5. Re: IP Subnetting Format
                  Kapila Withanawasam

                   

                  Hi Lee,

                   

                   

                  You are correct, the 10th subnet is 192.168.16.144

                   

                   

                  It is my mistake.

                   

                   

                  Kapila

                   

                   

                  • 6. Re: IP Subnetting Format
                    Lee Phillips

                     

                    Hi Kapila, Thanks for the reply. I thought I might of done something incorrect and just wanted to make sure I wasn't missing something. Once again thanks for all your help

                     

                     

                    Kind Regards

                     

                     

                    Lee