Default enabled command

Hi,

Command "ip subnet zero" (exclude quotes) is enabled on the router by default starting with cisco ios ver 12.x. This command allows you to use the first and last subnet in your network design. could you plz explain this in detail.

Thanks,

If you had an IP address range (say 10.12.14.x) carved up using a subnet mask of /30 (which lets you have just two hosts in each subnet and is used for point-to-point links) then there will be numerous subnets available for you to use in your design. The first one - for example 10.12.14.0 through 10.12.14.4 giving usable host addresses of 10.12.14.2 and 10.12.1.4.3 - traditionally wasn't accepted by Cisco kit as the range started with a zero. If you wanted to use that range you had to explicitly turn on the 'ip subnet zero' command which made that subnet usable. This however is now on by default

Hi,

Thanks for your reply, some more clarification
According to above example
IP address - 10.12.14.x
subnet mask - 255.255.255.252
so number of possible subnets - 64
subnets are - 0, 4, 8, ....252

Again back to example (for example 10.12.14.0 through 10.12.14.4 giving usable host addresses of 10.12.14.2 and 10.12.1.4.3 - traditionally wasn't accepted by Cisco kit as the range started with a zero)
Network Address - 10.12.14.0
First host - 10.12.14.1
Last host - 10.12.14.2
Broadcast address - 10.12.14.3
Again Network address starts - 10.12.14.4 and go on..

could you plz explain about ip subnet zero...

You're right; the first subnet is 0 through to 3 as you say. That's what I get for trying to fire off a quick reply while doing other stuff!
That first subnet is the 'zero subnet', the one affected by the 'ip subnet zero' command. If ip subnet zero is enabled then you can enter addresses from that subnet into the router and it will accept it. If ip subnet zero is not enabled then when you try to type in those values it won't accept them. I can't think of anything else to add really.

Hi,

i think, we can start assigning the values 10.12.14.1 and so on..because ip subnet zero is enabled by default
if it is not enabled then we need to start the subnet 10.12.14.4

am i right?