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Subnetting problem - Magic number

trikantejabuka
Level 1
Level 1

Hi,

This is my IP address 174.121.14.165/23

/23  - 255.255.254.0

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Magic number is: 256-254=2

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Networks:

0

2

4

6

8

10

12

14

16 ...

Network address: 174.121.12.0  

Broadcast address: 174.121.13.255        (12+Magic number -1 = 12+2-1 =13)

Where is mistake?

-------------------------------------------------------------------------------------------------------------------------

Please help me,

Greetings.

3 Accepted Solutions

Accepted Solutions

John Blakley
VIP Alumni
VIP Alumni

Joni,

Your math is fine, but the network address that you have on your interface (your IP) will start at 174.121.14.0 - 172.121.15.255.

HTH,

John

HTH, John *** Please rate all useful posts ***

View solution in original post

Joni,

The magic number, as you call it, is indeed 2 in the third octet of the netmask. Hence, you need to round down the third octet of the IP address to the nearest integral multiple of 2 when calculating the network address. However, in the IP address 174.121.14.165, the "14" is already an integral multiple of 2 so you do not change it. Right?

Best regards,

Peter

View solution in original post

Joni,

In addition to what Peter said, the "magic number" is considered your boundary. For example, if your address the magic number is 2. Multiples of 2 would be the starting network address for the next block:

172.121.2.0

172.121.4.0

172.121.6.0

etc.

The magic number is the starting boundary for that network, and the broadcast address is -1 less than the next starting boundary:

172.121.2.0 - 172.121.3.255 (broadcast) (256 - 1)

The useable addresses are calculated like:

254 (in the second octet) (11111110)

0 = There are 8 bits left in the 4th octet = 00000000

So together it's 11111110.0000000

There are 9 0s in the host portion of your mask, so you get 512 hosts per network. Then you take 512 - 2 leaving you with 510 useable addresses (172.121.2.1 - 172.121.3.254)

HTH,

John

HTH, John *** Please rate all useful posts ***

View solution in original post

6 Replies 6

lgijssel
Level 9
Level 9

Was your ip not in x.x.14.0?

John Blakley
VIP Alumni
VIP Alumni

Joni,

Your math is fine, but the network address that you have on your interface (your IP) will start at 174.121.14.0 - 172.121.15.255.

HTH,

John

HTH, John *** Please rate all useful posts ***

Thanks for the reply,


I know my IP address should be in the range 174.121.14.0 - 172.121.15.255, in my calculation

Network Address is: 174.121.12.0

Broadcast address is : 174.121.13.255, which is obviously wrong.


But I ask myself why, if I calculate correctly all.

Joni,

The magic number, as you call it, is indeed 2 in the third octet of the netmask. Hence, you need to round down the third octet of the IP address to the nearest integral multiple of 2 when calculating the network address. However, in the IP address 174.121.14.165, the "14" is already an integral multiple of 2 so you do not change it. Right?

Best regards,

Peter

Joni,

In addition to what Peter said, the "magic number" is considered your boundary. For example, if your address the magic number is 2. Multiples of 2 would be the starting network address for the next block:

172.121.2.0

172.121.4.0

172.121.6.0

etc.

The magic number is the starting boundary for that network, and the broadcast address is -1 less than the next starting boundary:

172.121.2.0 - 172.121.3.255 (broadcast) (256 - 1)

The useable addresses are calculated like:

254 (in the second octet) (11111110)

0 = There are 8 bits left in the 4th octet = 00000000

So together it's 11111110.0000000

There are 9 0s in the host portion of your mask, so you get 512 hosts per network. Then you take 512 - 2 leaving you with 510 useable addresses (172.121.2.1 - 172.121.3.254)

HTH,

John

HTH, John *** Please rate all useful posts ***

trikantejabuka
Level 1
Level 1

Thank you all for  help, now I understand

Thank you!.


greeting from Bosnia

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