Why is classful addressing obsolete?

Question

Two portions of my Question:

1ST PORTION

As the internet grew, the original Classful addressing scheme became a limitation. The IP address space was being exhausted. (the question is how)

2ND PORTION

In case of classful addressing scheme, all networks had to choose one of three possible sizes, many addresses were unused (this sentence requires explanation)

What Is my point of view about this sentence, for example we have two networks using class c addressing scheme, in this case we can have 254 hosts , but we need to have only 12 hosts with our network. It means we only need 12 IP addresses, now in the second network we need to have 254 hosts, now we assign the IP addresses in sequence to these hosts (as the same suffix can be assigned to the computers on different networks because there network numbers are different so the IP addresses are unique).
Teacher says that if you need only 12 hosts in your network and you use class c Addressing scheme then you need only 4 bits in your suffix, but in case of class C you will assign the suffix portion to 12 computers and all the remaining possible IP addresses will be wasted. Question is what is the problem if they are wasted, I think they are not wasted we can use them in another network as I have described above.

Please note that i am a beginer not an expert like you people, therefore answer should be such that can be understood.

Answer 1

Classful addressing, is simply a term used to describe how many hosts (aka the size of) are in a network segment, the newer term is slash-notation or CIDR. They mean roughly the same thing.

Heres a quick cheatsheet for converting them:

Classful Name    CIDR     subnet mask used
Class A          = /8     = 255.0.0.0
Class B          = /16    = 255.255.0.0
Class C          = /24    = 255.255.255.0

Obviously there are network segments of all sizes, see this wikipedia entry for more details.

Answer 2

WHY are they still teaching about class-based addressing? It's gone. Obsolete as you said. Your question has no real merit, but the reason the addresses are wasted is because if you give me 255 addresses and I only use 3 of them, the other 252 can ONLY be used by me. I can't give them away to anyone else (OK, so they're not "wasted" because I CAN use them at a later time, but only I can use them. Nobody else. And that's like me buying 1/50th of every car on the planet and then I can't let anyone else us them. That's a waste).

The internet is fastly approaching the day when we run out of IPv4 addresses (that's another topic for another day, some believe we're already there), so we can't afford any wasteful allocations.

Answer 3

In response to #2: They are wasted because of ROUTING. Not because of Addressing. If you are assigned a /24 network (Class C), Internet Service Providers will forward ALL traffic going to that entire /24 to YOUR router. Since you only have your 12 hosts behind your router, the rest goes to waste, since nobody on the internet could get to them. With CIDR and the allowance of smaller pools of hosts, you can route to 10.1.1.16/26, 10.1.1.64/27 (pulling those out of nowhere, they're probably not right) instead of having to route to 10.1.1.0/24.

Answer 4

This information might help you - http://www.tcpipguide.com/free/t_ProblemsWithClassfulIPAddressing.htm

Summary of “Classful” Addressing Issues There are three main problems with “classful” addressing, which are somewhat related to each other (making them a bit harder to explain). Let's start with a quick summary of what these issues are:

Lack of Internal Address Flexibility: Big organizations are assigned large, “monolithic” blocks of addresses that don't match well the structure of their underlying internal networks.

Inefficient Use of Address Space: The existence of only three block sizes (classes A, B and C) leads to waste of limited IP address space.

Proliferation of Router Table Entries: As the Internet grows, more and more entries are required for routers to handle the routing of IP datagrams, which causes performance problems for routers. Attempting to reduce inefficient address space allocation leads to even more router table entries. "

Answer 5

Question is what is the problem if they are wasted, I think they are not wasted we can use them in another network as I have described above.

In the standard classful networking scheme, the following is true:

• Boundaries that form address ranges are fixed
• The different subnet sizes are tied to the network part of the address

Because of this, let's say you have a classful /8 from someone. It's impossible for you to split that amongst smaller networks. You can't change the network part of the address to change the class (as long as others are using that network address to send you traffic), and therefore the range of addresses you are working with.

CIDR removes both these limitations and allows you to "subnet" a range of IP addresses down to as little as 2 addresses.