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Cisco router 2600 how to Check and set IP subnet - zero?


I have a cisco router 2600.
How do I set the router to "IP Subnet - Zero" ?
What does this mean?
How can I check the current setting?


- Jeff
1 Solution
Type, in config mode, "ip subnet-zero" to make the change. Newer versions of IOS have it on by default. To check, type "show running-config" and the config will say it if it's on. If it's not in the config, it's off.
router#show config

ip subnet-zero <-- enabled
no ip subnet-zero <== disabled

What does it do? Just what it says, it allows you to use "subnet zero". By strict RFC standards, you cannot use subnet zero or a broadcast subnet as host IP's.
For example, / 24 is subnet-zero of the 192.168.x.x class C address space.
   /24 is the broadcast subnet. By strict RFC standards, neither of these subnets are available for use, although they are widely used. On a strict standards-based Cisco router, you must expressly enable it to use those subnets. Mike is correct that on the most recent versions of IOS, it is enabled by default.

> For example, / 24 is subnet-zero of the 192.168.x.x class C address space.

No, IS the 192.168.0.x class C address range. is subnet "0" of the 16 possible /28 subnets within the 192.168.0.x class C address space.

When subnetting was introduced, there was some concern that was ambiguously the network address of both the subnet and the class C space.  So Cisco implemented rules which would not let you use the subnet (you could still use,, etc) unless you configured "ip subnet-zero" indicating that you had determined that it was safe to do so on your network.

As far as I know, nobody has ever actually had a problem with this, which is why this setting is ON by default in recent IOSes.

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