CCNA question...

[tahir1234]

Hi seniors, I have read so many articles on classful ip address vs classless ip address but could not get the idea. say there are two ip addresses as follows

1. 10.0.0.1 255.0.0.0

2. 10.0.0.1 255.255.0.0

Which address (above) is classful and which is classless. Lot of theory based on just classful and classless, so a clear concept is vital. Thanks for reply. Best regards. tahir

[tahir1234]

Hi Jon, No apologies please, everyone has the right to contribut into the forum, a very informative discussion is here that we new network persons cannot find elsewhere. I thanks for everyone who contributed into and hopefully will continue.

Regards,Tahir

[tahir1234]

Hi seniors, uptill yet i concluded from the discussion that 1.10.0.0.1 255.0.0.0 is classful whereas 2.10.0.0.1 255.255.0.0 is classless. I got this question during my study for ccna, and writer say second address is classless very reson that it is not on natural boundary as IOS will think. But my seniors say "it depends on how that address is being used, I mean then how IOS will know what situation is there, if anythink is defined in IOS then I am not aware of. Please guide me. Thnaks again to all seniors who participated in it and hopefully will further clear it.

[Jon Marshall]

Tahir

" But my seniors say "it depends on how that address is being used"

This is the point we have been discussing really and it is obvious that different people have different views so perhaps there is no definitive answer.

I would argue that when we talk about how the address is being used then we are really talking about classful vs classless routing protocols ie. RIP/IGRP = classful, OSPF/EIGRP/RIPv2 = classless.

But i still fundamentally agree with Joseph in that the address itself can be talked about as to whether it is classless or classful based on whether it uses the natural mask ie.

Class A = 255.0.0.0

Class B = 255.255.0.0

Class C = 255.255.255.0

HTH

Jon

[Joseph W. Doherty]

Hope this doesn't muddy the waters further, but since you also asked about concepts, wanted to touch on a few points.

For "book" answers, as you initially posted, they're likely looking for whether the given address and mask can be represented by a classful A/B/C IP address. Classful rules define certain masks for certain address ranges and are known as "natural" masks; natural because they are implied.

As Jon notes in his latest reply, some routing protocol are classful, e.g. RIPv1/IGRP, and some are classless, e.g. RIPv2/OSPF/EIGRP. What makes them different is the former only has passes along addresses, the latter passes along addresses and their masks. Because of this difference, how network topologies are designed for classful vs. classless addressing may differ. Classless can mimic a classful design, but can do more. The rules for using "natural" IP addresses differ from using "non-natural" IP addresses within a classful network. Both, though, can be used and usually were. (E.g. point-to-point links with /30 addressing.)

When thinking of classful vs. classless IP addressing then, we're really thinking within the framework of network topology and specific routing protocols. (As also noted by Jon.)

Beyond dealing with classful vs classless, and dealing with just IP addresses and masks, they don't have to conform to either. Two examples come to mind, first a host's perspective with a interface IP address and mask. Host's perspective isn't classful or classless, it's just and IP address and mask. (Perhaps one of the things Rick had in mind.) Second, we haven't touched upon static routing. Here too, you just see IP address and mask, it's left to you how to utilize them.