classeless vs VLSM vs CIDR

Technically speaking, Classless IP addressing is IP addressing without using the standard subnet mask.

That means 141.1.0.0 can use 255.255.255.0 if you wish.

Variable-length subnet mask is a technique in classless ip addressing that involves borrowing bits from the hosts id portion of a subnet mask to create subnet ids. Such subnet masks will be like 255.255.128.0 for example.

Classless inter-domain routing, is an application of VLSM (it uses VLSM). Meaning that routing between network segments does not rely on the standard subnet mask, but rather uses the VLSM or subnet id of the segment.

but I think in general, those terms are interchangable, the difference is slim.

hankooknara

actually, your explanation is very clear.

thank you!!!!

No problem. Good luck.

If you do not know yet:

http://www.learntosubnet.com is a very good source.

Also, if you do not have a background in networking, perhaps pursue Network+ first prior to taking your CCNA. It will give you a taste of what it will be like, plus prepare you with the fundamentals.[/url]

Your idea's are not correct, a classless protocol sends the mask in updates.It also means the strict adherance to the class a,b,c inital bits is no longer needed, this allows for the possiblity for cidr.
Classless protocols support vlsm which means the mask can vary throughout a major network number deployment.This means if an isp gives me a 100.x.x.x address i can vary the mask length throughout my network with 100.x.x.x/30,100.x.x.x/20 etc the mask is varying form /30 to /20 to /xx

bighornsheep wrote:

Technically speaking, Classless IP addressing is IP addressing without using the standard subnet mask.

That means 141.1.0.0 can use 255.255.255.0 if you wish.

This is incorrect, i can have a classful routing protocol with an address
151.78.1.1/25, in this case i must use /25 throughout my network when using a 151.78.x.x network address.

Variable-length subnet mask is a technique in classless ip addressing that involves borrowing bits from the hosts id portion of a subnet mask to create subnet ids. Such subnet masks will be like 255.255.128.0 for example.

Vlsm is when the mask varies,it is not about subneting.

Cidr is when you move the network boundardy to the left rather than to the right.Another name is supernetting.A class c address natural mask is /24 so imagine 192.168.1.0/24, if i want to create a cidr block i can supernet this address and use 192.168.0.0/16, its like summarization but since i go beyond the natural /24 mask i.e i use /16 its called cidr.

theseman

Heres how I look at it:

CIDR (Subnet mask remains consistent throughout sub-networks)

Your ISP gives you 211.16.21.0 /24. You want 2 networks, each with equal # of hosts (62). In this case you would use CIDR, since the subnet mask will be the same for each sub-network. (255.255.255.192). Howerver, there are limitations of CIDR. It only allows a mask up to /27, so if you have a subnetwork that only requires a few hosts IP's, there will be some going to waste!

VLSM (mask can vary throughout sub-networks)
VLSM allows you to break the bits down further, so you can theoretically have a network with a mask of /30. Useful if you had a sub-network that only required 2 host IPs. If you are given the same IP of 211.16.21.0 /24, and you had 2 networks that required a different amount of hosts (one with 62 and one with 2), you could set it up as follows:
62 hosts subnet-work - mask of 255.255.255.192
2 host sub-network - mask of 255.255.255.252

I know this was long but I hope it helps!!

Travis

theseman wrote:

Heres how I look at it:

CIDR (Subnet mask remains consistent throughout sub-networks)

You explained vlsm correct but not cidr.Your explanation of cidr is actually
flsm.Read my post above!

bighornsheep wrote:

Technically speaking, Classless IP addressing is IP addressing without using the standard subnet mask.

ed_the_lad wrote:

Your idea's are not correct, a classless protocol sends the mask in updates.It also means the strict adherance to the class a,b,c inital bits is no longer needed, this allows for the possiblity for cidr.

Didnt we just say the same thing?
I think I have the right idea about VLSM and Classless IP though.

I'm sorry about my explanation of CIDR, I didnt know that was supernetting.

bighornsheep wrote:

Didnt we just say the same thing?
I think I have the right idea about VLSM and Classless IP though.

You left out the important part,

Classless protocols support vlsm which means the mask can vary throughout a major network number deployment.This means if an isp gives me a 100.x.x.x address i can vary the mask length throughout my network with 100.x.x.x/30,100.x.x.x/20 etc the mask is varying form /30 to /20 to /xx

bighornsheep wrote:

Technically speaking, Classless IP addressing is IP addressing without using the standard subnet mask.

So what you say by the above statement is a classless protocol is ip addressing without using the standard subnet mask.
You seem to think the class A address with a /8 , a class b with /16 and a class c with /24 are the only masks allowed with a classful routing protocol.The rule you must follow for classful protocols is the mask must remain constant per major network number,also the mask cannot be smaller than the standard mask i.e. a class A must be >= 8, class B >= 16 and class C >= 24.
The reason i'm correcting you is because you've written a similar incorrect
response in another thread i didnt bother correcting, now i've decided to

bighornsheep wrote:

A classful network will only use 255 or 0 in the subnet octet but 0 must be the last octet, so 255.0.255.0 is not a valid subnet mask. This will allocate a set number of hosts to every subnet, and VLAN is the (I think the only) way to seperate broadcast domains.

A classless network is basically a flexible form of the above, and you can use the custom subnet masks to specify subnets and number of hosts in them.

Here is the reponse for the other thread i mentioned above!
Dont get the wrong idea,i'm not breaking your balls, just pointing you in the right direction.

ed_the_lad wrote:

You left out the important part,

You seem to think the class A address with a /8 , a class b with /16 and a class c with /24 are the only masks allowed with a classful routing protocol.The rule you must follow for classful protocols is the mask must remain constant per major network number,also the mask cannot be smaller than the standard mask i.e. a class A must be >= 8, class B >= 16 and class C >= 24.
The reason i'm correcting you is because you've written a similar incorrect
response in another thread i didnt bother correcting, now i've decided to
.

I really didnt know that. So you're saying classful IP addressing, a class A IP address can still use 255.255.255.0 as its subnet mask?

bighornsheep wrote:

I really didnt know that. So you're saying classful IP addressing, a class A IP address can still use 255.255.255.0 as its subnet mask?

Yes

theseman

I just read what your posted about CIDR above, and now wonder if I need to do some reading.

I did not think supernetting and CIDR were the same thing. Supernetting to me is combining multiple networks into one "super" network.

i.e. You have 3 class C's from your ISP, but you need "lots" of hosts in one big network. You combine the 3 class C's and are left with one big network.

ed_the_lad wrote:

Cidr is when you move the network boundardy to the left rather than to the right.Another name is supernetting.

Supernetting is another thing... supernetting is called summarization. You use summarization where you have two adjacent networks and what to use just one entry in the routing table. ie:

networks 192.168.0.0 and 192.168.0.32 (mask 255.255.255.224 or /27) are reachable thru S0/0. You can create a summarization entry like this:

ip route 192.168.0.0 255.255.255.192 inteface s0/0

CIDR is used with some routing protocols and is normally used by ISPs to assign blocks of IP addresses. Instead of saying "your network is 192.168.200.0 255.255.255.0, your net is 192.168.200.0/24"

agustinchernitsky wrote:

Cidr is when you move

networks 192.168.0.0 and 192.168.0.32 (mask 255.255.255.224 or /27) are reachable thru S0/0. You can create a summarization entry like this:

ip route 192.168.0.0 255.255.255.192 inteface s0/0

THis is summarization, 192.168.0.0 is a class c address,using classful routing the first three bytes are for the network and these are reserved automatically when the system see's the ip address has the first 3 bits
110 which represents a class c network.
So we have 192.168.1.0/24, if i want to summarize this network i can use 192.168.0.129/25,192.168.0.5/30 this is called summarization and nothing to do with supernetting.If however i move the summarization to the left of the major bit boundard so the mask is less than /24 it is called superneting or another name cidr. /22 will make 4 class c blocks.

agustinchernitsky wrote:

CIDR is used with some routing protocols and is normally used by ISPs to assign blocks of IP addresses. Instead of saying "your network is 192.168.200.0 255.255.255.0, your net is 192.168.200.0/24"

So you think by just changing the mask from 255.255.255.0 to /24 something has changed and it is now cidr? The /24 is a representation that the first bits in an ip address are no longer important, so the address is no longer constrained by the classful mask,which means with cidr i can summarize a class C address to the left of the 24 bit standard mask, i.e.
/23,/22,/21.. etc, cidr or superneting
/25,/26,/27.. etc, summarization

If you want to continue to think summarization and supernetting are the same thing thats your choice i suppose, but its better not to send others down the same path!

Hi Ed,

First of all: I am studdying for CCNA... I'm not an expert. What I post is What I learnt so far.

Second: Sybex CCNA Study guide 5th ed by Todd Lammle:

If you’re confused about what summary routes are, let me explain. Summarization, also called supernetting, provides route updates in the most efficient way possible by adverting many routes in one advertisement instead of individually.

Well, Todd thinks it has something to do with supernetting. A sample he gives in his book:

For example, if you wanted to summarize the following networks into one network advertisement, you just have to find the block size first, then you can easily find your answer:
192.168.16.0 through networks 192.168.31.0
What’s the block size? There are exactly sixteen Class C networks, so this neatly fits into a block size of 16.
Okay, now that you know the block size, you can find the network address and mask used to summarize these networks into one advertisement. The network address used to advertise the summary address is always the first network address in the block—in this example, 192.168.16.0.
To figure out a summary mask, in this same example, what mask is used to get a block size of 16? Yes, 240 is correct. This 240 would be placed in the third octet—the octet where we are summarizing. So, the mask would be 255.255.240.0.

In other words... network 192.168.16.0/24 gets to summarized as 192.168.16.0/20 (I don't like it in this format, but...)

Regarding CIDR... yes, I wasn't so clear... so, to clear things out, let us Todd enlighten us:

Another term you need to familiarize yourself with is Classless Inter-Domain Routing (CIDR).
It’s basically the method that ISPs (Internet Service Providers) use to allocate an amount of addresses to a company, a home—a customer.
When you receive a block of addresses from an ISP, what you get will look something like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation (/) means how many bits are turned on (1s).

Maybe I wasn't clear in my answers or maybe Todd is giving us the wrong concepts...

Besides, look at your comment:

So we have 192.168.1.0/24, if i want to summarize this network i can use 192.168.0.129/25,192.168.0.5/30 this is called summarization and nothing to do with supernetting

That would be subnetting... not supernetting / summarization.

And finally:

If you want to continue to think summarization and supernetting are the same thing thats your choice i suppose, but its better not to send others down the same path!

Ed... Buddy, we are here to help. Its in my best interest to post the most accourate information on my knowledge. Still, I am human (really

) and I can make mistakes... This is why I took the time to paste info from a well recognized author for CCNA prep guides.

Thanks anyway for your post.

So we have 192.168.1.0/24, if i want to summarize this network i can use 192.168.0.129/25,192.168.0.5/30 this is called summarization and nothing to do with supernetting

That would be subnetting... not supernetting / summarization.

Yes, that mistake is due to a long post.

My example of summarizing should have been,
you have afew networks 192.168.1.0/30,.4/30,.8/30,.12/30 summarized to 192.168.1.0/28, as you can see /28 > /24 i.e. summarization.
Todd explans very basic as this is for beginners and he doesnt want to overwhelm, but the problem is he breeds confusion.I explained the difference
between the two, if you are unsure you should seek a second or third source and not come back and say i am wrong.If you dig deeper into the understanding of this you will realise my explanation is correct.
P.s. maybe my writing style comes off harsh but i dont have time to add cute fluffy statements to make you feel good, i'm just stating facts and thats all

, there a smile now you will feel happy eh? haha

Well Ed, you are right, I should have re-checked before posting

And yes... Remember Todd prepares you for CCNA... not CCNP, so his explanations although they are good... are basic.

Well buddy, nice talking with you... see you soon.

ps: Thanks for the smile! Now I feel better! LOL!

hankooknara

This is going into my CCNA notes for good now.. Please someone correct me if I am wrong.. thanks.

Classful (FLSM support classful addressing)
When you're routing in a classful network, there is no need to send the subnet mask along with your IP address because the routing protocol you are using will look at the first octet, determined the class and assume the mask (ie, 255.0.0.0 for Class A, 255.255.0.0 for Class B and 255.255.255.0 for Class C).
If address starts with 195.3.2.1/26, this is still classful since 26 >= 24
For example, let's say we've assigned 197.22.75.128/26 to a subnet. If we're using RIPv1, which is a classful, FLSM routing protocol, then we can't assign 197.22.75.96/28 as another subnet

Classless
Classless means you're using a routing protocol that makes no assumptions about class. It does not look at the first octet and then applies a subnet mask. Instead, the classless routing protocol will pass that information along with the IP address.

VLSM
VLSM allows for multiple subnet masks so an the organization is able to assign
different extended-network-prefix to different subnets as required by each
subnetwork, not just the single prefix you'll see in FLSM.
Bear in mind that VLSM was, and is, still essentially classful.

CIDR, or "supernetting"
CIDR uses the network prefix, instead of the first octet, to determine the dividing point between the network number and host number.
So my guess of difference between VLSM & CIDR is that vlsm is still falls under classful and CIDR is based on classless.

I wouldnt go putting this in your notes yet!

Classful,it doesnt send a mask with the update,if the receiving interface shares the same major network number it will assume to use the mask of the receiving interface,otherwise it will be summarized to the classful boundary.

Vlsm is not classful and never was.Classful protocols need a fixed length mask.

Cidr has been discussed in this thread so you should read again.

hankooknara

http://www.cramsession.com/articles/files/how-long-is-your-subnet-m-9162003-1313.asp?

according to that site.. it says VLSM is classful to a degree and i think i understand..

but thank you for your correction.. it really helps me..!

Hello,

well I have a doubt now with this... Ed help me out. This is what I know (or think I know):

RIPv1 is a classfull routing... in other words, it won't advertize subnet masks. So, if you have 195.3.2.1/26 all your network shoud be using the same subnet mask.

Some comments con classfull and classless:

Classful network: uses the same subnetmask on all the network. According to CCNA study guide, if you use a /26 on all your network, it is considered classful.

Classless network: you use different subnets on your network... probably you are using VLSM.

Classful routing protocols: the don't advertize the subnet mask, so you need to use the same subnet on your network. Think of RIPv1 and IGRP.

EDIT: Just got this from google:

Classful routing is a consequence of the fact that routing masks are not advertised in the periodic, routine, routing advertisements generated by distance vector routing protocols. In a classful environment, the receiving device must know the routing mask associated with any advertised subnets or those subnets cannot be advertised to it. There are two ways this information can be gained:

• Share the same routing mask as the advertising device
• If the routing mask does not match, this device must summarize the received route a classful boundary and send the default routing mask in its own advertisements

Classless routing protocols: they do advertize the subnetmask... so you can use 195.3.2.1/26 or VLMS. Think of RIPv2, EIGRP and OSPF.

Obliged

hankooknara wrote:

http://www.cramsession.com/articles/files/how-long-is-your-subnet-m-9162003-1313.asp? according to that site.. it says VLSM is classful to a degree and i think i understand...

Any other good links out there on this subject? besides http://www.learntosubnet.com

agustinchernitsky wrote:

Hello,

well I have a doubt now with this... Ed help me out. This is what I know (or think I know):

RIPv1 is a classfull routing... in other words, it won't advertize subnet masks. So, if you have 195.3.2.1/26 all your network shoud be using the same subnet mask.

Some comments con classfull and classless:

Classful network: uses the same subnetmask on all the network. According to CCNA study guide, if you use a /26 on all your network, it is considered classful.

Classless network: you use different subnets on your network... probably you are using VLSM.

Classful routing protocols: the don't advertize the subnet mask, so you need to use the same subnet on your network. Think of RIPv1 and IGRP.

EDIT: Just got this from google:

Classful routing is a consequence of the fact that routing masks are not advertised in the periodic, routine, routing advertisements generated by distance vector routing protocols. In a classful environment, the receiving device must know the routing mask associated with any advertised subnets or those subnets cannot be advertised to it. There are two ways this information can be gained:

• Share the same routing mask as the advertising device
• If the routing mask does not match, this device must summarize the received route a classful boundary and send the default routing mask in its own advertisements

Classless routing protocols: they do advertize the subnetmask... so you can use 195.3.2.1/26 or VLMS. Think of RIPv2, EIGRP and OSPF.

Hi agustinchernitsky,
The link provided by hankooknara is not exactly correct,what you have typed above is good and you understand correctly.If you read through this thread it will explain everything you need as far as im concerned.If
hankooknara wants to believe everything he reads on that link thats up to him,just dont let it confuse you,it talks only about summarizing at the major net and not about using the mask of the received interface if the major net is the same.I'm only offering information at the end of the day,
if someone wants to believe something else thats their business.