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assign all 0s all 1s network address on an interface

Dear All...

I have read this Article regarding ip-subnet-zero command in Cisco …...

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Prior to Cisco IOS 12.0, by default, Cisco routers wouldn't allow you to configure an IP address on the all 0s network on an interface. However, you could configure this using the ip subnet-zero command in Global Configuration Mode.

Now, after IOS 12.0, the ip subnet-zero command is the default on routers. Note that this command not only allows the all 0s subnet, but it also permits the all 1s subnet. And that's why you no longer have to subtract 2 when using the network formula.
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Is this meant that I can configure an interface with the following address :

192.168.1.0 255.255.255.252
192.168.1.3 255.255.255.252

Or with :

192.168.1.252  255.255.255.252
192.168.1.255  255.255.255.252

Also :
With  can I configure subnet mask of 255.255.255.254 on Cisco routers…..

Thanks at advance ..
 

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abu_deep
Asked:
abu_deep
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2 Solutions
 
Don JohnstonInstructorCommented:
192.168.1.0 255.255.255.252
No. This is a network address of the first subnet (all 0's in the host field)

192.168.1.3 255.255.255.252
No. This is the broadcast address of the first subnet (all 1's in the host field)

192.168.1.252  255.255.255.252
No. This is a network address (all 0's in the host field)

192.168.1.255  255.255.255.252
No. This is the broadcast address of the last subnet (all 1's in the host field)

Now what you could do is:
192.168.1.1 255.255.255.252
and
192.168.1.2 255.255.255.252

Which is an all 0's subnet that you can't do without subnet zero.

You can also do:
192.168.1.253 255.255.255.252
and
192.168.1.254 255.255.255.252

Which is an all 1's subnet.
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abu_deepAuthor Commented:
what about the  255.255.255.254 subnet I have seen it before on cisco routers........
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lrmooreCommented:
In the "old days" subnet zero would not be allowed.
An example would be
192.168.0.0/24
Or even your example of 192.168.1.0/30
Nor would the last subnet be allowed:
 192.168.255.0/24
 192.168.1.252/30
Following the old rules, neither of these subnets were supposed to be used
You could not use anything in the subnet 192.168.0.x, nor assign IP's 192.168.1.1-192.168.1.2/30 because they are part of subnet zero

Most modern IP stacks don't care, but Cisco IOS is more RFC compliant so the command to allow those subnets had to be added and is now the default.

As Don points out, you still can't use all zeros as a host IP, but you can use all the valid host ips within the subnet.

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Don JohnstonInstructorCommented:
The only way to have a "subnet" of .254 would be if you had a /31 mask. That would leave a host field on 1 bit which is not allowed.
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