1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

auto summary and vlsm problem

Discussion in 'General Cisco Certifications' started by kobem, Dec 10, 2006.

  1. kobem

    kobem Megabyte Poster

    791
    1
    50
    men, i know what vlsm and auto summary is but i have questions
    about them..

    we know that ripv1 doesn't support discontiguous networks
    also vlsm but ...

    for example i have three networks in my hand

    one of them is 172.16.10.0/24 the other is 10.10.0.0/30
    and the other one is 172.16.20.0/27

    with /30 is between the others and the serial(link) one
    i assume we use ripv1 , if so , can we communicate these three?

    second one : ripv1 doesn't support discontiguous networks
    so
    am i wrong here that discontiguous network is (we have networks like for example 172.16.20.0 and 172.16.30.0
    and between them there is a different network?

    third one : we have 172.16.10.0/24, 10.10.0.0/30 ,172.16.20.0/24 can they talk with ripv1?

    forth one : we have 172.16.10.0/24, 10.10.0.0/24 ,172.16.20.0/24 can they talk with ripv1?

    last question : if i have five different networks , in this case
    can i use vlsm or auto summary(supernetting) ?

    i have confusion where i use supernetting or vlsm
     
    Certifications: CCNA
  2. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    No to all your RIPv1 questions - RIPv1 doesn't support VLSM. 172.16.x.x always always always uses /16 when VLSM isn't used.
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  3. NetEyeBall

    NetEyeBall Kilobyte Poster

    279
    10
    45
    Kobem...To research futher you might want to look at classfull versus classless routing protocols. Rip 1 is a classfull routing protocol which is why it doesn't support VLSM. It only understands the classfull boundries ie Class A with mask of /8, Class B with a mask of /16, and Class C with a mask of /24.

    Rip2 is classless and supports VLSM.
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall
  4. kobem

    kobem Megabyte Poster

    791
    1
    50
    what is discontiguous network ?

    i mean for example i have 172.16.1.0/16 , 192.168.1.0/24 ,
    10.1.0.0/8 is this thing discontiguous?

    or 172.16.1.0/16 , 172.16.2.0/24 , 172.16.3.0/24 ?

    or 172.16.1.0/16 , (between the 172s) 10.1.0.0/16 ,172.16.2.0/16 ? (for rip and ripv2)

    and rip doesn't support vlsm and can't use it but if i have
    rip where should i use auto-summary?


    if we think ripv2 that is classless one and it supports
    vlsm also auto-summary and if i have this network
    172.16.0.0/24 what should i do?(vlsm or auto summary)
     
    Certifications: CCNA
  5. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    If you can't put the network ranges side by side and have an unbroken string of IP addresses, the networks are discontiguous... meaning, not contiguous.

    Obviously, 172.16.1.0/16 , 192.168.1.0/24, and 10.1.0.0/8 are discontiguous. (One word of caution, though: 172.16.1.0/16 isn't a valid network address. It should be 172.16.0.0/16 or 172.16.1.0/24.)

    If you've got 172.16.0.0/16 and 172.17.0.0/16 and 172.18.0.0/16 and 172.19.0.0/16, you've got a contiguous network of addresses. But if you're missing 172.17.0.0/16, then your network will NOT be contiguous.

    Similarly, 172.16.1.0/24 (NOT /16, be careful...), 172.16.2.0/24, and 172.16.3.0/24 are contiguous.
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  6. Spice_Weasel

    Spice_Weasel Kilobyte Poster

    254
    45
    45
    Actually, a discontiguous network is one in which parts of the same classful network are seperated by a different network. RIP v1 and IGRP do not support VLSM and as such cannot handle discontiguous networks. OSPF, RIP v2, EIGRP, IS-IS, etc support VLSM and can therefore handle discontiguous networks.

    For example, imagine a company uses two networks, 10.1.1.0/24 and 10.1.2.0/24. These networks are part of the same classful network (10.0.0.0/8 ). If those two networks were connected by a WAN link which used 172.22.60.4/30, that would be a discontiguous network, as two parts of the same classful network are seperated by a different network. If RIP v1 was used as a routing protocol it would be unable to distinguish between the two networks as it would only have a single, classful entry (10.0.0.0/8 ) in its routing table for the two networks. Routing protocols that support VLSM solve this problem as they advertise and understand classless addresses. A router running OSPF would have two entries in its routing table, one for 10.1.1.0/24 and one for 10.1.2.0/24, enabling the router to properly route packets to those networks.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  7. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    ...not necessarly, if that OSPF router sees both routes through the same interface. In that case, it is possible to summarize them. Since those two ranges ARE contiguous, you could summarize those two routes to 10.1.0.0/22. However, since you don't have administrative control over 10.1.0.0/24 and 10.1.3.0/24, you'll be misadvertising those ranges.

    I can better explain by using your example, but using 10.1.0.0/24 and 10.1.1.0/24. Even if separated by a 172.16.0.0/16 network, if another OSPF router saw both routes through the same interface, both networks would still be summarized to 10.1.0.0/23.

    EDIT: I guess we're both saying the same thing... it just depends on where you are in the network to determine whether a network is contiguous or not.
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  8. kobem

    kobem Megabyte Poster

    791
    1
    50
    thanks for the answers but i also have some new questions
    (i kinda like stupid :cry: )


    "One word of caution, though: 172.16.1.0/16 isn't a valid network address. It should be 172.16.0.0/16 or 172.16.1.0/24."

    first what dou mean with invalid?


    second question : i see in books for example we have 192.168.10.0/24 network and we vlsm it and with this
    man i show the picture :


    here why we use just 192.168.10.0/24 network ?(don't we need
    172 network or 10 network here?



    third question : again a picture


    here why don't we use vlsm ?

    can't i use different networks and in which cases is this
    possible?

    [Edit-Bluerinse - removed copyrighted pictures]
     
    Certifications: CCNA
  9. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    Before I begin, I must tell you that those pictures are copyrighted, and shouldn't be reproduced here without the permission of the author.

    The questions you're asking require a very basic understanding of networking. I mean no disrespect, but trying to understand route summarization without already knowing what constitutes a valid network address or knowing when you should use one address range over another, is sort of like trying to run before you can walk.

    First question:
    172.16.1.0/16 is not a valid network address; it's a valid host address. The network address is the first address in a range, the broadcast address is the last address in a range, and host addresses are all the other addresses in a range. The host address 172.16.1.0/16 is on the 172.16.0.0/16 network. The /16 means that 16 bits are used for the network portion of the address. That's the 172.16 part. The first address in the 172.16.0.0/16 network is 172.16.0.0. The broadcast address is 172.16.255.255. Everything else, including 172.16.1.0, is a host address on that network.

    Second question:
    The following networks are RFC 1918-compliant addresses (previously RFC 1597):
    10.0.0.0/8 (10.0.0.0 - 10.255.255.255)
    172.16.0.0/12 (172.16.0.0 - 172.31.255.255)
    192.168.0.0/16 (192.168.0.0 - 192.168.255.255)
    Any address range within these networks can be used for private networks. It doesn't matter which range you use. You aren't required to use all of them - you can just use a piece of one. For example, if your network is small and has less than 254 hosts on it, you can simply use 192.168.100.0/24. Or 192.168.2.0/24. Or 10.0.1.0/24. The choice is yours. So in your picture, you don't need a 172. or 10. network. You can have one... but why?

    Third question:
    You ask why don't we use VLSM. But we are using VLSM. 10.0.0.0/16 is a subnet of the original 10.0.0.0/8 range (as is 10.1.0.0/16, 10.2.0.0/16, etc.). Route summarization takes ALL those variable length subnet masks and summarizes them all together into one range. Word of warning: if you don't understand how to subnet address ranges, then learning route summarization isn't going to make any sense to you.

    Hope this helps! And remove those pictures from your post!
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  10. kobem

    kobem Megabyte Poster

    791
    1
    50
    for my first question(about invalid network address)

    you said that 172.16.1.0/16 is not valid network address
    but valid host address and

    i ask if we make it 172.16.0.0/24 then we use ripv1 and
    have 172.16.1.0/24 , 2.0/24 and 3.0/24 so

    1.0/24 is here why is this valid network address?
    i think according to this (172.16.0.0 -172.31.255.255)
    it is not valid network address again?


    second one : it is /24 then if we use ripv1 here
    /24 shows us vlsm here? and if we don't use vlsm here
    how could i distribute ips(according to what?)
     
    Certifications: CCNA
  11. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    172.16.1.0/24 is a valid network address because it is the first address in the range 172.16.1.0 to 172.168.1.255. Keep in mind that when you say /24, you mean that 24 bits (first three octets) are used for the network portion of the address and 8 bits (last octet) are used for the host portion of the address. Thus, 172.16.1 is the network portion, and 0 is the host portion. Since 0 is the first address in the range, it is the network address. 255 is the broadcast address, and 1-254 are client addresses.

    The full range 172.16.0.0 -172.31.255.255 doesn't use a /24 mask... it's a /12 mask. A /24 mask is a very small piece of the /12 mask. If you use a /12 mask, then 172.16.1.0 is not a valid network address... only 172.16.0.0 would be a valid network address if a /12 mask is used.

    If this is confusing, or you don't understand why, you *really* need to learn how to subnet before moving further with routing. Entire chapters have been written on how to subnet.
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  12. kobem

    kobem Megabyte Poster

    791
    1
    50
    did you see my second question?
     
    Certifications: CCNA
  13. kobem

    kobem Megabyte Poster

    791
    1
    50
    [​IMG]

    look at here 172.17.0.0 , 172.18.0.0 and mask is 255.255.0.0
    i mean default subnet mask so how can we assign
    these networks here?(they are not valid.)

    and last question when we use 172.17.1.0/24 and we use ripv1 how could we distribute ips to interfaces because i can't subnet
    with ripv1?
     
    Certifications: CCNA
  14. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    Yes, I saw your second question, and I said that unless you go back and learn subnetting, the routing portion won't make any sense. But I'll go ahead and answer these next questions...

    The 172.17.0.0/16 and 172.18.0.0/16 networks ARE valid. Why do you believe they are not? If you don't understand why they are valid, you *really* need to learn subnetting.

    You cannot use 172.17.1.0/24 masks properly with RIPv1 becaues RIPv1 doesn't add subnet mask information in the routing packets. 172.17.1.0 is a Class B address, which will be treated by RIP as a /16 mask. Only Class C addresses can use a /24 mask with RIPv1.

    Not sure what you mean by "distribute ips to interfaces". You assign IP addresses to interfaces manually. Do you mean routes?
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  15. kobem

    kobem Megabyte Poster

    791
    1
    50
    hey i know you hate me and i have been eating my head at this time

    [​IMG]

    here is rip routing example but there is something i didn't understand here

    by default class c addressing is in bits network portion.network portion.network portion. host portion (in ip address)

    and here there is ripv1 and it must have all same subnetworks
    but lik i said above 192.168.10.0 and 192.168.20.0 are different networks because 10 and 20 are network portion so how does
    this work ?

    (other thing is that they are contiguous but not same subnetworks)
    (in ripv1 do thay have to be same subnetworks(parts of same network i mean or contiguous?)

    second question: i don't know this whether i asked here before
    but i ask now , if we have this 192.168.10.0/27 network
    without vlsm we could calculate
    subnets again and we find networks 192.168.10.32 , 64, 96 , 128,160,192,224 then we distribute but always with
    /27 am i right?

    if we could use vlsm and we have serial link requires
    2 hosts we could give /30 notation for this link its block size
    4 after that we distribute others like 192.168.10.32/27 and
    192.168.10.64/27 ok?
     
    Certifications: CCNA
  16. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    19,136
    462
    374
    I don't hate you at all... I just want to be sure that you have a solid understanding of subnetting and IP addressing, and what is and isn't allowed with classless and classful protocols.

    WARNING: Be advised that you're posting copyrighted information without the publisher's permission. You can't copy information from e-books or scanned pages or even type word-for-word what you see from a book, or you're violating UK and International Copyright Law. You should remove your graphic.

    Here are your answers:

    Yes, default Class C is network.network.network.host.

    RIPv1 doesn't have to have the same subnetworks. RIPv1 will simply advertise the two routes, 192.168.10.0 and 192.168.20.0. No route summarization can occur.

    You can't use /27 with RIPv1 and expect it to route properly. You can only use the classful subnet mask boundary. For a Class C, that's /24. Again, RIPv1 cannot use VLSM.

    192.168.10.0 uses a 24-bit mask with classful protocols like RIPv1. If you use a different routing protocol (or none at all), you can subnet this network into:
    192.168.10.0/25 and 192.168.10.128/25
    or
    192.168.10.0/26, 192.168.10.64/26, 192.168.10.128/26, and 192.168.10.192/26
    or
    192.168.10.0/27, 192.168.10.32/27, 192.168.10.64/27, 192.168.10.96/27, 192.168.10.128/27, 192.168.10.160/27, 192.168.10.192/27, and 192.168.10.224/27
    ...etc.
    So yes, you are correct that you can split the 192.168.10.0/24 range up (if you're using a classless protocol to advertise these routes).

    Yes, if you could use VLSM, you can split up the /24 range into as many pieces as you want. Not all have to use the /27 range, for example. So you are correct - you can take your /30 serial link from the 192.168.10.0/27 range, then use the remaining /27 ranges (starting with 192.168.10.32/27, as you stated) for other networks. You could squeeze in 8 of those /30 ranges using that initial 192.168.10.0/27 block, if you wanted to:
    192.168.10.0/30
    192.168.10.4/30
    192.168.10.8/30
    192.168.10.12/30
    192.168.10.16/30
    192.168.10.20/30
    192.168.10.24/30
    192.168.10.28/30
    That's the magic of VLSM... you're not stuck using the same subnet mask for each subnet, and you're not wasting address space.
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!

Share This Page

Loading...