Review Questions. Answer 17, page 171

[pastrin]

You state that if ip subnet-zero is not active we have only 6 subnets avalaible instead of 8.

I think there shoud be 7 subnets available. If we have 8 and we drop the first one (zero net) that leaves us with 7.

Thanks in advance

[ciscodaze]

This is because along with the subnet 0 (all-zero's subnet), the so called all-one's subnet (the subnet that contains the broadcast address) was also not used...at least, they discouraged it, back in the day (of classful routing) but engineers still used both subnets by configuring ip subnet zero on their routers.

So with ip subnet zero (being off by default, until IOS version 12) the first and last subnets in any range of subnets were not used.

This is all legacy stuff. Cisco has a really good short page on this topic.
http://www.cisco.com/en/US/tech/tk64...80093f18.shtml

[bobdempsey]

The all-ones subnet was also generally included in the no-no list. It was understood that if the network in question prohibited the all-zeros subnet, it also prohibited the all-ones subnet.

All of this has been obsolete since the mid '90s.

There is a lovely discussion at http://www.cisco.com/en/US/tech/tk64...80093f18.shtml

[pastrin]

Thanks for your clarifications. Very interesting the link.