Classful vs Classless

shanparamesshanparames Posts: 87Member ■■□□□□□□□□
Hi

Please explain which criteria differentiate a classful addressing from classless ip addressing!


151.78.1.1/25 ip address is a classless ip address or classful ip address?


I found an interesting discussion in this forum from: http://www.techexams.net/forums/ccna...m-vs-cidr.html

In this discussion, I found the statement as follows:

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.


So I need which factor either the first few bits of an ip address or the subnet mask decides it's type classful or classless?

Thanks

Swaminathan
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Thanks,

Regards

S.Swaminathan

Comments

  • phoeneousphoeneous Go ping yourself... Posts: 2,333Member ■■■■■■■□□□
    For the purpose of the CCNA, classful addressing uses Class A, B, or C subnet masks and classless doesnt. So with classful addressing, you will have networks like 10.0.0.0/8, 172.16.0.0/16, and 192.168.1.0/24. In classless addressing, you will have networks like 10.0.0.0/10, 172.16.0.0/22, and 192.168.1.0/30. Classless uses the concept of vlsm, or variable length subnet mask.

    To answer your question, 151.78.1.1/25 is a classless ip address.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    phoeneous wrote: »
    For the purpose of the CCNA, classful addressing uses Class A, B, or C subnet masks and classless doesnt. So with classful addressing, you will have networks like 10.0.0.0/8, 172.16.0.0/16, and 192.168.1.0/24.

    Sorry, don't mean to nitpick, but....... Classful networks may be notated as such, but for the most part, things are classful because they don't contain the subnet mask in their updates, the subnet mask is assumed based on what bits are flipped on in the first octet. Using CIDR notation when discussing classful networks can be a little misleading.

    Long story short, classful networks have either predetermined network prefixes, or make assumptions based on the interface configuration.

    Classless does away entirely with the concept of classful networking. Rather than using the bits in the first octet to automatically assume a subnet mask, the amount of bits used to mask that prefix are sent with the prefix as a requirement.

    What it basically boils down to is that, in classful networking, the prefix is inferred by the router. In classless networking, the prefix is explicitly defined by the router sending the update.
  • shanparamesshanparames Posts: 87Member ■■□□□□□□□□
    Hi Phoeneous and Forsaken!

    Thanks a lot for both you to give the solutions.

    Quote:
    Originally Posted by bighornsheep
    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.



    I found this text in one of the post in this forum which i posted before which I found from this link:


    http://www.techexams.net/forums/ccna-ccent/15813-classeless-vs-vlsm-vs-cidr.html

    My question is whether it is possible to have the classful routing protocol with an ip address 151.78.1.1/25 ?

    Please advise me

    Thanks

    S.Swaminathan




    Thanks,

    Regards

    S.Swaminathan
  • phoeneousphoeneous Go ping yourself... Posts: 2,333Member ■■■■■■■□□□
    Sorry, don't mean to nitpick, but.......

    No need to apologize, If I say something wrong I'd rather someone point it out and tell me than me giving other people wrong information. Thanks for the clarification.
  • gosh1976gosh1976 Posts: 441Member

    My question is whether it is possible to have the classful routing protocol with an ip address 151.78.1.1/25 ?

    Please advise me

    Thanks

    S.Swaminathan




    I would say yes.

    As a proof of concept I set up three routers connected together and put two pc's on the end routers. I set up RIPv1. I took a class A address space: 10.0.0.0 but I used a /16 subnet mask. the middle router has a routing table like this:

    Gateway of last resort is not set

    10.0.0.0/16 is subnetted, 4 subnets
    R 10.0.0.0 [120/1] via 10.3.0.3, 00:00:12, FastEthernet0/0
    R 10.1.0.0 [120/1] via 10.2.0.1, 00:00:15, FastEthernet0/1
    C 10.2.0.0 is directly connected, FastEthernet0/1
    C 10.3.0.0 is directly connected, FastEthernet0/0

    However, If I take one of the subnets in the middle and make it a /30 in order to conserve space then I am using VLSM and I no longer have connectivity and can't ping from pc to pc. If I then change the routing protocol to RIPv2 - The routing tables are rebuilt and I can ping again.

    Perhaps someone with a bit more experience could verify what I'm thinking.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    The /25 is valid as long as all the routers in the network use it. If all the interfaces share the same subnet mask, the router will use that instead of summarizing to the classful boundary.
  • shanparamesshanparames Posts: 87Member ■■□□□□□□□□
    Hi


    Now I understood the concept of classful and classless routing protocols related to ip addressing either it is classful or classless used in the networks involved in routing .

    Gosh ! I appreciate your clear example with the routing table given while using rip1.And the explanation is clear that the pc's connected to the ends of the extreme end routers pinging with each other (after insertion of vlsm concept) configured with rip2 which is a classless protocol

    Once again thanks a lot

    S.Swaminathan
    Thanks,

    Regards

    S.Swaminathan
  • sthompson86sthompson86 Posts: 370Member
    Kinda going along with the OP's question.

    Does CISCO CCNA always assume that network bits will always be class full? As in a Class A will have 8 Network Bits Class B 16 Bits and so on?


    Like for example if I have a class A 10.20.68.2 with a 255.255.0.0 Sub I can assume that, that network will have no matter what 8 network bits?

    From what I can tell they do.

    - Thanks
    Currently Reading: Again to Carthage - CCNA/Security
  • alan2308alan2308 CISSP, MCSA 2008, MCSA 2012, CCNA R&S, CCNA Security Ann Arbor, MIPosts: 1,854Member ■■■■■■■■□□
    From what I've seen of Cisco questions, both in the Academy quizzes and exams, and the actual CCNA, there is nothing to assume.
  • sthompson86sthompson86 Posts: 370Member
    alan2308 wrote: »
    From what I've seen of Cisco questions, both in the Academy quizzes and exams, and the actual CCNA, there is nothing to assume.

    Thanks @.gif
    Currently Reading: Again to Carthage - CCNA/Security
  • Kelvin WangKelvin Wang Posts: 11Member ■□□□□□□□□□
    gosh1976 wrote: »
    I would say yes.

    As a proof of concept I set up three routers connected together and put two pc's on the end routers. I set up RIPv1. I took a class A address space: 10.0.0.0 but I used a /16 subnet mask. the middle router has a routing table like this:

    Gateway of last resort is not set

    10.0.0.0/16 is subnetted, 4 subnets
    R 10.0.0.0 [120/1] via 10.3.0.3, 00:00:12, FastEthernet0/0
    R 10.1.0.0 [120/1] via 10.2.0.1, 00:00:15, FastEthernet0/1
    C 10.2.0.0 is directly connected, FastEthernet0/1
    C 10.3.0.0 is directly connected, FastEthernet0/0

    However, If I take one of the subnets in the middle and make it a /30 in order to conserve space then I am using VLSM and I no longer have connectivity and can't ping from pc to pc. If I then change the routing protocol to RIPv2 - The routing tables are rebuilt and I can ping again.

    Perhaps someone with a bit more experience could verify what I'm thinking.

    Hi, pardon my ignorance, I am not able to understand what this test prove. It means RIPv1 does not support VLSM and RIPv2 does?

    why are there 4 subnets in 10.0.0.0/16?
    I have learned class B and class C subnetting, is this consider class A subnetting?
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    Hi, pardon my ignorance, I am not able to understand what this test prove. It means RIPv1 does not support VLSM and RIPv2 does?

    why are there 4 subnets in 10.0.0.0/16?
    I have learned class B and class C subnetting, is this consider class A subnetting?

    The point is to prove that classful does not always equals a preset mask.

    10.0.0.0, in a classful network, would normally carry a 255.0.0.0 subnet mask.

    But that's not a hard and fast rule. If all of the interfaces use the same subnet mask, the routers will infer the subnet mask from the one configured on the interface. But if any of them were different, it will then automatically summarize to the nearest classful boundary
  • gosh1976gosh1976 Posts: 441Member
    Hi, pardon my ignorance, I am not able to understand what this test prove. It means RIPv1 does not support VLSM and RIPv2 does?

    why are there 4 subnets in 10.0.0.0/16?
    I have learned class B and class C subnetting, is this consider class A subnetting?

    It is true RIPv1 is a classfull routing protocol so it does not support VLSM. RIPv2 is a classless routing protocol so it supports VLSM.

    In the RIPv1 example I borrowed 8 bits for the subnet mask so yeah I suppose you would call it class A subnetting. Part of the point was that the concept holds true for Class B or Class C addresses. In classfull subnetting the subnet mask must be the same for every subnet. The other point was that when I changed one of the subnets to a /30 I lost connectivity between the PC's and the routing table only showed the directly connected subnets. Then when I changed the routing protocol to version 2 the routing tables showed 4 subnets again and I could ping again.

    There are 4 subnets in 10.0.0.0/16 because I designed a little site with 4 subnets. As you can see in the diagram there are 4 subnets. Each interface on a router connects to a different subnet. Those happen to be fastethernet interfaces connected with crossover cables.
  • alan2308alan2308 CISSP, MCSA 2008, MCSA 2012, CCNA R&S, CCNA Security Ann Arbor, MIPosts: 1,854Member ■■■■■■■■□□
    When a /24 isn't wasteful enough for a point to point link...
  • Kelvin WangKelvin Wang Posts: 11Member ■□□□□□□□□□
    gosh1976 wrote: »
    It is true RIPv1 is a classfull routing protocol so it does not support VLSM. RIPv2 is a classless routing protocol so it supports VLSM.

    In the RIPv1 example I borrowed 8 bits for the subnet mask so yeah I suppose you would call it class A subnetting. Part of the point was that the concept holds true for Class B or Class C addresses. In classfull subnetting the subnet mask must be the same for every subnet. The other point was that when I changed one of the subnets to a /30 I lost connectivity between the PC's and the routing table only showed the directly connected subnets. Then when I changed the routing protocol to version 2 the routing tables showed 4 subnets again and I could ping again.

    There are 4 subnets in 10.0.0.0/16 because I designed a little site with 4 subnets. As you can see in the diagram there are 4 subnets. Each interface on a router connects to a different subnet. Those happen to be fastethernet interfaces connected with crossover cables.

    Hi,

    Thanks for the explanation. I got a clearer picture now.
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