Custom subnetting help?

[8uhbbhu8]

Alright so im taking a networking course in college and right now we are working on subnetting. the problem is no matter who explains it i just cant grasp custom subnet masks. Maybe some of you can help?

 

example:

 

# of needed subnets: 2000

# needed useable hosts: 15

Network Address 178.100.0.0

 

Address Class: B

Default Subnet Mask: 255.255.0.0

Custom Subnet Mask: 255.255.?.?

Total number of subnets: ____________

Total Number of hosts borrowed: _______________

Number of useable Addresses: _______________

Number of bits borrowed: __________________

 

               

 

                                        178   .  100   . 00000000 . 00000000

                                       IIIIIIII  . IIIIIIII  . 00000000 . 00000000

 

 

 

 

So im not sure where i should go from here. I know that the equation ((2^X) -2)) somehow works its way in but not sure exactly how or when. None of my materials help and the profs explanations are jibberish to me because he doesnt offer any reasoning on why it works the way he shows it and how to figure out what to use when and what each step does and why it works that way.

 

Any help would be greatly appreciated!

 

Thanks

[8uhbbhu8]

Quick addon:

 

I know that the subnet mask and the addresses that are filled in are represented by 8 1s. As well as that in order to get the rest of the mask you need to use the concept of borrowing the bits (0s) and however many of those bits you borrow correspond to the binary values that you add? together to make up the total number of hosts?

[dark_xzyph3r]

Quick addon:

 

I know that the subnet mask and the addresses that are filled in are represented by 8 1s. As well as that in order to get the rest of the mask you need to use the concept of borrowing the bits (0s) and however many of those bits you borrow correspond to the binary values that you add? together to make up the total number of hosts?

 

http://www.subnet-calculator.com/ during my CCNA class we used this tool to figure out what subnet to use based on how many users we had.

[8uhbbhu8]

The calculator would help with my worksheets but unfortunately we are required to show work and not allowed to use any tools during tests. thats why im asking the question.

 

http://www.subnet-calculator.com/ during my CCNA class we used this tool to figure out what subnet to use based on how many users we had.

[dark_xzyph3r]

The calculator would help with my worksheets but unfortunately we are required to show work and not allowed to use any tools during tests. thats why im asking the question.

 

Ahh, I see If I knew how to actually do the work I would help but i don't

 

This might help break it down more.. http://www.tcpipguide.com/free/t_IPSubnettingStep3DeterminingTheCustomSubnetMask.htm

 

You can use this to find the 1st 2nd 3rd and 4th octet to go along with that table.. http://www.tcpipguide.com/free/t_IPDefaultSubnetMasksForAddressClassesABandC.htm

Edited October 15, 2015 by dark_xzyph3r

[gazabi]

Alright so im taking a networking course in college and right now we are working on subnetting. the problem is no matter who explains it i just cant grasp custom subnet masks. Maybe some of you can help?

 

example:

 

# of needed subnets: 2000

# needed useable hosts: 15

Network Address 178.100.0.0

 

Address Class: B

Default Subnet Mask: 255.255.0.0

Custom Subnet Mask: 255.255.?.?

Total number of subnets: ____________

Total Number of hosts borrowed: _______________

Number of useable Addresses: _______________

Number of bits borrowed: __________________

 

               

 

                                        178   .  100   . 00000000 . 00000000

                                       IIIIIIII  . IIIIIIII  . 00000000 . 00000000

 

 

 

 

So im not sure where i should go from here. I know that the equation ((2^X) -2)) somehow works its way in but not sure exactly how or when. None of my materials help and the profs explanations are jibberish to me because he doesnt offer any reasoning on why it works the way he shows it and how to figure out what to use when and what each step does and why it works that way.

 

Any help would be greatly appreciated!

 

Thanks

I'll try to help. The formula works for since ip addresses uses binary, this gives the 2 in the 2^x. The x is the number of bits in the binary number, in one octet there are 8 (the 8 zeros). So 2 to the power of any number gives the total amount of different combinations or different addresses/networks. If you have 3 zeros or bits then it is 2^3 which comes out to 8, so 8 different possibilities. Now for the minus 2(used for calculating hosts not networks). The minus 2 is put there because in a range of addresses, the first is the network address and last is a broadcast address and  these cannot be used for the hosts.

 

Now for your problem. First I would get the total number of bits or x needed to get at least 15. Through using the formula given I would need 5, 4 is too small since it gives me only 4 usable addresses (remember the minus 2: 16-2=14). The 5 gives me 32 host addresses. The host bits are the last bits so it will be the last 5 zeros:

 

                                   178   .  100   . 00000000 . 00000000

                                  IIIIIIII  . IIIIIIII  . 00000000 . 000|00000|

                                                                                   |Host Bits|

 

Next is the network bits. We can use the same formula but without the minus 2. Since we have 16 zeros or bits and 5 of them were used in the host addresses this leaves 11 for the network bits. We need to get an x amount of bits which would give  us at least 2000 network possibilities. Using the equation 2^x I figured that the remaining 11 bits is just enough, giving us 2048 possible network addresses so we know that the 5 bits we borrowed for the users is just enough.

 

  178   .  100   . 00000000 . 00000000

 IIIIIIII  . IIIIIIII  . 11111111 . 111|00000|

                         |     Network Bits      |Host Bits|

 

 

Since the default subnet mask was 255.255.0.0 and we borrowed 11 bits from the host portion(all that was zero was host) we know that 8 bits is 255 for the third octet (255.255.255.x) now for the remaining 3 we know it is 224 since the bits have a value from(left to right) 128,62,32,16,8,4,2 and 1, the leftmost 3 bits add up to 224. So the new subnet mask is 255.255.255.224. The usable hosts were calculated to be 30, the subnets/ networks were calculated to be 2048, and we borrowed 11 of the 16 zeros or bits for the network. The only thing I am not sure about is the hosts borrowed since I have never encountered that before.

[dark_xzyph3r]

I really need to go for my CCNP cert lol

[gazabi]

I really need to go for my CCNP cert lol

I had the opportunity to take it but didn't bother since I didn't plan on going into networking.

[dark_xzyph3r]

I had the opportunity to take it but didn't bother since I didn't plan on going into networking.

 

Yeah I want to go into networking lol, I did the CCNA cert and got it but really need to do the CCNP so I can actually get a job with it

[8uhbbhu8]

.

 

 

 

Thanks for the answer. I think ive got it now. Your explanation was he first ive seen that makes sense to me.

 

Ill be posting what i have for steps on here later to flesh it out and maybe help others as well in the future

 

Thanks for those that responded and helped me.