Passed CCNA1 1

[cyberspace]

Took the CCNA1 exam yesterday, got 81%, so combined with the full marks I got on the practical exam, my total score for CCNA1 is 86.9%!

So pretty chuffed with that!

Although I actually did better on the CCNA1 exam....there were lots of Class B subnetting questions, and I don't really know it too well.

I want to make sure I don't get caught out again, so what good resources are there for Class B subnetting? I am fine with Class C, but doing 2 to the power of 14 in your head isnt exactly easy lol!

But never mind, I am inspired with confidence at the result and hope it continues through the program

'When all else fails.......read the manual'

 

[cyberspace]

edit: I meant I did better on the practice exam

'When all else fails.......read the manual'

 

[CiscoGuy33]

cyberspace,

This is a free website (most of it is free) with tons on subnetting.

http://www.learntosubnet.com/

I hope this helps!


E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[jdel]

Also, if you want to use something to check your work, Solarwinds has a great, free, downloadable subnet calculator.

 

[cyberspace]

Thanks for the links!

i'm getting the grasp of it now.

What made me think it was strange was how you can have say:

255.255.240.0 as the mask, providing 16 subnet IDs (14 if not using subnet zero) which provides 4094 hosts per subnet ID. I had it stuck in my head that the maximum was 253 hosts but thats becase you have less bits for Class C!

Hopefully in future I will now be able to accurately tackle Class B subnetting questions

'When all else fails.......read the manual'

 

[cyberspace]

oh i also meant to ask....will a class B subnet mask ALWAYS end with a 0? This also confused me before

eg 255.255.240.0?

'When all else fails.......read the manual'

 

[KiscoKid]

Yes pretty much. Because you are manipulating the 17th and 23rd bits in the 32 bit mask, the last octet (bits 25 to 32) remain untouched. Thing to remember is that the host portion of the subnet mask is always set to zero.

Regards

 

[Cluebird]

No, a Class B subnet mask will not always end with zero. It is not uncommon to see VLSM used with what started as a class B IP address. The first bit boundary is usually pushing the address to a 24 bit mask (255.255.255.0) and then further pushing the bit boundary to fit the network sizes. For example point-to-point links will get either a /30 (255.255.255.252) or /31 (255.255.255.254) if the IOS supports the /31. Other networks will get addressed based on their sizes so you can see just about any mask used (255.255.255.128; 255.255.255.192; 255.255.255.224; and so forth). You can also see Class B supernetting with a mask such as 255.0.0.0 or 255.128.0.0 or 255.192.0.0 for instance. About the only thing we look for these days regarding addresses classes is as the starting point for the mask if we need to subnet or supernet.

 

[wizo]

But would that not then be considered a B class ip address with a C class subnet mask?????

;-)

 

[cyberspace]

How can you have a /31? That doesnt have any usable addresses does it?

'When all else fails.......read the manual'

 

[Cluebird]

In the newest IOS Cisco lets you use a /31 on point-to-point links. And yes, that means you're using the network and broadcast addresses as host addresses. It is one of the efficiency mechanisms Cisco implements.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t2/ft31addr.htm

As far as a "B class ip address with a C class subnet mask" if that's the way you choose to look at it and it works for you, then think of it that way. However, technically it would be a B class ip address with 8 subnet bits. The Class C mask starts with a /24 before subnetting and is tied to the addresses that have a bit pattern 11000000 in the first octet and address pattern of N.N.N.H.