Stupid question Im sure

gonzoflick

I am getting the hang of subnetting thanks to the three step process learnt with CBT Nuggets videos. On these videos they say that to calculate the number of networks possible its just 2^x where x is the number of subnet bits. To find the # hosts its (2^x)-2 where x is # of host bits. So for 255.255.255.192 it would be 2^2 or 4 subnets, and (2^6)-2 or 62 hosts per subnet.


Well I'm now going over my Cisco press book that shows the same formula for finding hosts (2^2)-2

but the subtract 2 when finding total number of networks as well. IS THIS CORRECT? so instead of 255.255.255.192 being 2^2 = 4 its (according to Cisco press) (2^2)-2 or 2? I understand why the first and last address of the host portion is unusable due to network ID and broadcasting but why do they subtract from the network number too?

Any help will be appreciated as these two sources differ in their information and I know how important a topic this is.

miller811

This explains it better than I could, hope it helps.


Subnet-Zero
Using subnet zero for addressing was discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.

With reference to our example above, consider the IP address 172.16.1.10. If you calculate the subnet address corresponding to this IP address, the answer you arrive at is subnet 172.16.0.0 (subnet zero). Note that this subnet address is identical to network address 172.16.0.0, which was subnetted in the first place, so whenever you perform subnetting, you get a network and a subnet (subnet zero) with indistinguishable addresses. This was formerly a source of great confusion.

Prior to Cisco IOS® Software Release 12.0, Cisco routers, by default, did not allow an IP address belonging to subnet zero to be configured on an interface. However, if a network engineer working with a Cisco IOS software release older than 12.0 finds it safe to use subnet zero, the ip subnet-zero command in the global configuration mode can be used to overcome this restriction. As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default, but if the network engineer feels that it is unsafe to use subnet zero, the no ip subnet-zero command can be used to restrict the use of subnet zero addresses.

In versions prior to Cisco IOS Software Release 8.3, the service subnet-zero command was used.

The All-Ones Subnet
Use of the all-ones subnet for addressing has been discouraged in the past because of the confusion inherent in having a network and a subnet with identical broadcast addresses.

With reference to the example above, the broadcast address for the last subnet (subnet 172.16.224.0/19) is 172.16.255.255, which is identical to the broadcast address of the network 172.16.0.0, which was subnetted in the first place, so whenever you perform subnetting you get a network and a subnet (all-ones subnet) with identical broadcast addresses. In other words, a network engineer could configure the address 172.16.230.1/19 on a router, but if that is done, he can no longer differentiate between a local subnet broadcast (172.16.255.255 (/19)) and the complete Class B broadcast (172.16.255.255(/16)).

Although the all-ones subnet can now be used, misconfigurations can cause problems.

Using Subnet Zero and the All-Ones Subnet
It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0. Even prior to Cisco IOS Software Release 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.

On the issue of using subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.