# IP Subnetting confusion subnet number incrases

Member Posts: 55 ■■□□□□□□□□
Have an address 10.16.3.65/23 with a Network ID of 10.16.2.0. These are the tricky ones the ones that increase the subnet digit and not the host digit. You know what I mean all the easy ones they do on youtube.com where deal with subnets /25 and above
But besides that this is the difficult part of subnetting crossing over to the next digit in the subnet mask.

Show me how 10.16.2.0 is the network ID of 10.16.3.65 (when I thought it was 10.16.3.0) the subnet range is 512 host per subnet with 128 subnets

• Member Posts: 579 ■■■■□□□□□□
With a /23 mask you're subnetting in the 3rd octet. With 8 network bits which are fixed (Class A network /8 mask) this means you have 23 - 8 = 15 bits for subnetting. 8 of those 15 bits are in the 2nd octet (.16.) which remains fixed because all the bits in this part of the mask represent the network. So we focus on subnetting in the 3rd octet. So we have in essence 16 "fixed" bits with the rest be used for subnetting 23 - 16 = 7 bits. With 7 bits for subnetting we have a subnet mask of 254 which means our subnets IDs increment by 2 (Block size = 256 - Subnet Mask = 256 - 254).

Our subnets are then:

10.16.0.0 <- Zero Subnet
10.16.4.0

Address ranges are 10.16.2.1 - 10.16.3.254 with a broadcast address of 10.16.3.255. So the address 10.16.3.65 is within the range of 10.16.2.0 - 10.16.3.255.

You can also think of a /23 as combining two /24's together.
• Member Posts: 423
AndreL wrote: »
...Show me how 10.16.2.0 is the network ID of 10.16.3.65 (when I thought it was 10.16.3.0) the subnet range is 512 host per subnet with 128 subnets

I always look at the underlying binary so that I can 'see' what's going on.
```IP Address.. 0 0 0 0 1 0 1 0 | 0 0 0 1 0 0 0 0 | 0 0 0 0 0 0 1 1 | 0 1 0 0 0 0 0 1 = 10.16.3.65
Mask........ 1 1 1 1 1 1 1 1 | 1 1 1 1 1 1 1 1 | 1 1 1 1 1 1 1 [COLOR=#ff0000]0 [/COLOR]|[COLOR=#ff0000] 0 0 0 0 0 0 0 0[/COLOR] = 255.255.254.0
'AND'....... 0 0 0 0 1 0 1 0 | 0 0 0 1 0 0 0 0 | 0 0 0 0 0 0 1 [COLOR=#ff0000]0 [/COLOR]|[COLOR=#ff0000] 0 0 0 0 0 0 0 0[/COLOR] = 10.16.2.0
```
The 'bits' that I've highlighted in RED are the 'host bits'.
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