Subnetting confusion

[azev]

Which method does cisco use for subnetting? Is it ((2^N)-2) or (2^N) ?
The reason I asked; I remember a while ago when I took my CCNA it was using ((2^N)-2) method. Just now I found some website that gave me new information about cisco is using the (2^N) method these days.
http://www.mcmcse.com/articles/subnetting123.shtml
http://www.mcmcse.com/articles/subnetting.shtml
I am planning to retake my test next week, and just curious which method should I apply when answering test questions.

Thanks

 

[randal]

Depends on how the question is posed, honestly. If they ask for how many usuable, use (2^N)-2, if they just ask for 'how many', use (2^N). Thats always worked for me.

 

[azev]

make sense, but the probem i'm having is to determind which one is the first ip address of the usable subnet. Lets say a class C network 192.168.81.0 subnetted with a 224 subnet or /27. What would be the first ip address of the usable subnet ? is it 192.168.81.1 or 192.168.81.33 ?

 

[Cooky]

It depends on when you use it. If you're taking a Cisco exam, it's 192.168.81.33. If it's real world situation where subnet-zero is implemented, it's 192.168.81.1.

 

[err]

this is very interesting. Why would cisco use the 1st IP as 192.168.81.33 ? not .1 ?

A quick dnsstuff.com cidr calculator shows:

CIDR range 192.168.81.0/27
Netmask 255.255.255.224
Wildcard Bits 0.0.0.31
First IP in range 192.168.81.0 (network address)
Last IP in range 192.168.81.31 (broadcast address)
First useable IP in range 192.168.81.1
Last useable IP in range 192.168.81.30
Number of useable IPs in range 30

if you really create a subnet of 192.168.81.0/27, I would think 192.168.81.33 would live in subnet 192.168.81.32/27

 

[randal]

Right, but I believe that Cisco's exam does not assume the `subnet-zero` option; hence any subnet that has it's network IP trailing with .0 is unusable. Nobody does that in the real world though, as it would be incredibly wasteful, IP-wise.

 

[err]

i see. So if it is 192.168.81.32/27, the gateway would also be 192.168.81.33 ?

and if it is 192.168.81.64/27, the gateway would be 192.168.81.65 ?

 

[randal]

Yep. I've personally always used the first usable IP as the gateway, although it is a common practice to use the last available IP (62 & 94 in your scenarios).

 

[err]

yes, first available IP seems to be the standard in the real world. Thanks for the enlightment