Classful vs Classless Subnetting

satishtech

Novice question:

Is there a difference between classful and cidr subnetting ?

Example :
10.0.0.0/16

10.0.0.0 10.1.0.0

---

so on (Lammle Book)
10.0.0.1 10.1.0.1
10.0.255.254 10.1.255.254
10.0.255.255 10.1.255.255

In Classless subnetting does the second Octet stay constant and we only get
10.0.0.0
10.0.0.1
10.0.255.254
10.0.255.255

I am confused is there a difference ?

Simrid

If my understanding is correct (someone correct me if i'm wrong) but - Classful subnetting is the "normal" subnet which is used for the class network.

Class A: /8
Class B: /16
Class C: /24

The Cider notation is simply just a way to represent the subnet. You CAN still have a classless subnet but use cider to represent it - For example /30 etc.

This is useful say in VSLM, where you don't want to "waste" any addresses.

This article on Wikipedia may help, there's a nice table a little down the page which is useful.

http://en.wikipedia.org/wiki/Classful_network

I hope this helps.

Node Man

I think the idea becomes more clear when we understand why we subnet a classless network. We use classless subnetting to conserve address space.

ccie14023

I wish classful addressing would disappear from certifications and training materials. It hasn't been used in a long time. I remember answering this very question on Groupstudy.com in 2004 and people are still having to learn it.

Simrid is correct that there are certain "natural" masks associated with different classes of addresses. With a classful address, you could tell what part of the address what network and what part was host simply by looking at the fist octet. So if the first octet was anywhere between 1 and 127, you knew it was class A, and therefore the first octet was network and the last three octets host, basically /8. If it was between 128 and 191, you knew it was half network, half host, or /16. In the old days we used to request a "class C" from our ISP, which meant we were looking for 255 (more or less) addresses.

Where it got kind of confusing was that if you were assigned an address block, say 190.1.0.0, which is class B, you could subnet it down further. Eventually this whole system went away in favor of just specifying the mask always, which is what we have now.

I'm all for teaching obsolete technologies when they help folks understand current technologies, but classful addressing just confuses the heck out of people. Sorry you still have to study it.

networker050184

My thoughts exactly ccie14023. Time to just forget about classful addressing. Maybe a quick history blurb but no point in teaching it in practice anymore.

satishtech

so there is a difference,confusing,I guess I must understand and know both.
I am using this link hope that is enough for the ccent exam
IP Subnet Practice Page | Lammle.com

Priston

With classless subnetting you can be given a address of 10.121.76.0/24. Even though it's a "Class A" subnet it's still a /24.

With classful subnetting if someone mentions 10.121.76.0, it's an address in the 10.0.0.0/8 network.

See why no one uses it?

ccie14023

Look, the basic idea is that part of the address is "network" and part of it is "host". So a /24 is N.N.N.H, right? The last octet is host. Normally we need a subnet mask to tell us which bits are network and which bits are host. The idea behind classful addressing is that you can tell which bits are network and which are host without needing a mask. How do you know? By the value of the first octet. Depending on what range it is in, the network/host divide moves. So a class A has one octet (8 bits) of network and 3 of host, whereas a class C is 3 octets of network and one of host. Now what makes it really confusing is that you can subnet down a classful address. Where it gets kind of interesting is how a router deals with routes in a classful environment. Just do "no ip classless" on a Cisco router and see what happens.

I haven't seen the CCENT exam, but my guess is that if you study classless addressing and understand it, then you probably could just memorize what range A, B, C, and D addresses are, and what their "natural mask" is.

Why does nobody use it? Because it really doesn't add anything except unneeded complexity. A subnet mask really carries all the information you need and is far more flexible. Really the only way we use the classes is to define multicast addresses, which are class D, but this is not a routing thing so much as a pre-defined, reserved range of addresses.

satishtech

Priston : CCENT exam will I be asked lucidly in the question's either classful or classless.
or should I presume classless.

Am using this site IP Subnet Practice Page | Lammle.com
along with lammle book and odom's book for subnetting hope thats enough.

Fulcrum45

satishtech wrote: »

Priston : CCENT exam will I be asked lucidly in the question's either classful or classless.
or should I presume classless.

Am using this site IP Subnet Practice Page | Lammle.com
along with lammle book and odom's book for subnetting hope thats enough.

Good question. It escapes me now but I recall seeing sample questions before that did not specify whether I was using classful or classless and made the whole task that more difficult. I would like to think that by now we should expect classless but classful never seems to go away.