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Why can't I use 255.255.255.128 ??

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15 replies

by frankl27

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Hello, everyone!

Here's a simple question, I hope!

I need to create two subnets. Everything I understand suggests I'd mask it using 255.255.255.128, but everyone I talk to tells me no. Isn't this right:

255.255.255.0 (one subnet: 1-254)
255.255.255.128 (two subnets: 1-127, 129-254)
255.255.255.192 (four subnets: 1-63, 65-127, etc..)

15 replies

by frankl27

Oh, I should probably point out that I need this because I have two floors and ~100 machines on each floor.

by dklippert

As far as I know, as long as your router recognizes the mask,

RFC 1878 allows 255.255.255.128

ftp://ftp.rfc-editor.org/in-notes/rfc1878.txt

Though MS still says it's unsupported:

Class C Subnetting Options for RAS Routing Q139983

http://tinyurl.com/2d0u

by lordinfidel

You are doing it correctly.

let's say you are subnetting 192.168.1.0

Your IP sturcture would be:

Netwk ID: 192.168.1.0
Mask: 255.255.255.128
Hosts: 192.168.1.1-126
Brdcst: 192.168.1.127

Netwk ID: 192.168.1.128
Mask: 255.255.255.128
Hosts: 192.168.1.129-254
Brdcst: 192.168.1.255

But now you also have to think about routing. Now that you have subnetted that range, how is 192.168.1.129/25 going to talk to 192.168.1.1/25?

You will need a router (can be a linux box or a nt box with ip forwarding enabled) connected to both netwks for both subnets to be able to talk to each other.

by frankl27

I'm noticing that you're referring to Class A, B or C. How can I tell which class address I'm using and whether or not I'll be able to use this mask?

by beskow

If you are using a class "C" address with the 255.255.255.128 subnet mask, there is just one bit used for subnetting (128). This violates one rule of subnetting: the subnet value cannot be zero or all ones. If you are using a Cisco router it is OK if you have version 12.0 or newer IOS. With 11.3 to 12 use the subnet zero command to use these two address. Prior to 11.3 you cannot use the zero or all ones subnet.

by nettek

Two subnets:
Subnet 0 192.0.0.1-192.0.0.126
You cannot have 192.0.0.127, because the bit mask would look like
11111111.11111111.11111111.11111111
which is for broadcasting

Subnet 1 192.0.0.129-192.0.0.254
You cannot have 192.0.0.255, becausethe bit mask would look like
11111111.11111111.11111111.11111111
which is for broadcasting

by mbatty

The previous answers describe the subnet issue very well.

The Class of a network is determind by the value of the first two bits. This means that you can tell what Class a network is by the value of the first part of the aaddress.

Class A 1-127
Class B 128-191
Class C 192-254
Class D 255 - Braodcast and Multicast addresses

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