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Can we assign the same ip address to two different hosts on two different subnets with different subnet masks?

Aug 26, 2009 2:57 AM

Abdur Rehman 2 posts since
Aug 26, 2009

I am a CCNA student and was practicing VLSM. I have subnetted a network but I got a common range of IP addresses in two different subnets. I searched the net and people say if you have different subnet masks it won't give a conflict. But in my case even with different custom subnet masks, I am getting a conflict.

 

Also there is a command called ip subnet zero. What is all that about? Is it sumthing related to this?

 

Thanks in advance!

  • Cristian Matei - CCIEx2 331 posts since
    Jul 8, 2008

    The command "Ip subnet-zero" has nothing to do with your issue. If you need further explanation on this command let me know.

     

     

    Regards,

    Cristian.

  • Winston Ritson 9 posts since
    Sep 10, 2008

    Hi,

     

    If you have 2 different subnets then there should be no IP Address overlap.

     

    Example:

     

    Subnet 1

     

    192.168.0.0/25 Broadcast 192.168.0.127

     

    Subnet 2

     

    192.168.0.128/25 Broadcast 192.168.0.255

     

    Thus with those two /25 subnet there can be no overlap of IP Addresses. Please show your subnets and we can take it from there. IP Classeless allows the router to ignore classful boundaries.

  • Conwyn 7,907 posts since
    Sep 10, 2008

    Hi Abdur

     

    We could Network Address Translation which would translate them to something else.

     

    Regards Conwyn

  • Chris 811 posts since
    Jul 25, 2008

    Hello Abdur,

     

    When using VLSM you can definitely have networks with different SNM's which overlap their IP Host range with each other.  It's very important to write out all the subnets of the "chosen" custom SNM and their host ranges.  With VLSM you usually start with your largest or smallest subnet size and begin creating your subnets ensuring no address overlap as you continue the process.

     

    HTH

  • Cristian Matei - CCIEx2 331 posts since
    Jul 8, 2008

    Hi vlanmlps,

     

       How do yo come up with networks with different SM which overlap their IP  host range?

    For example take 11.0.0.0/8.

     

    Cristian.

  • Paul Stewart  -  CCIE Security, CCSI 6,989 posts since
    Jul 18, 2008

    At this point, I am a not completely sure of the question.  I think if we had an example we could work through it.  I do think whatever it is the short answer will be it's not possible.  However with NAT and VRF a lot of things are possible, but would be well beyond the scope of which I think the question is being asked.

  • Conwyn 7,907 posts since
    Sep 10, 2008

    Hi Paul

     

    Very clever I had forgotten you can have VRF on a standalone router.

     

    Regards Conwyn

    !
    ip vrf NetworkA
    rd 81:81
    !
    ip vrf NetworkB
    rd 82:82

    !

    interface FastEthernet0/0
    ip vrf forwarding NetworkA
    ip address 10.0.0.1 255.255.255.0
    duplex auto
    speed auto
    !
    interface FastEthernet0/1
    ip vrf forwarding NetworkB
    ip address 10.0.0.1 255.255.255.0
    duplex auto
    speed auto

     

    Router#show ip int brief
    Interface                  IP-Address      OK? Method Status                Protocol
    FastEthernet0/0            10.0.0.1        YES manual up                    up

    FastEthernet0/1            10.0.0.1        YES manual up                    up

  • Cristian Matei - CCIEx2 331 posts since
    Jul 8, 2008

    Hi Conwyn and Paul,

     

      While what you're saying is correct, stick at the CCNA level and at the scope of the question. Try not the make things convoluted.

     

     

    Regards,

    Cristian.

  • Conwyn 7,907 posts since
    Sep 10, 2008

    Hi Cristian

     

    Welcome to CLN. CLN is over one year old but we have always tried to answer questions simply but equally stimulate the CLN membership whether by the history of the technology, the range of technology or real world application. We also discuss errors in the learning material. People may assume there are jobs which exactly relate to the Cisco certifications but in reality one operates in some areas where one might have great knowledge and others where one has none. It is important that people  develop skills which allows them to operate in a low knowledge environment.

     

    Regards Conwyn

  • Cristian Matei - CCIEx2 331 posts since
    Jul 8, 2008

    Hi Conwyn,

     

     

      I stick to my previous statement. For example, if i add a question in the CCNA section, this means i'm currently at that level, without any advanced knowledge. So if you really want to help people out, answer their questions without adding too much overhead of unkown stuff to them.

     

    Regards,

    Cristian.

  • Chetan 539 posts since
    Jul 26, 2008

    Hi Abdur,

     

    When practicing VLSM,  It often happens with me that when I have worked on many VLSM questions (smaller ones), I start doing more of mental calculations, rather than using pen and paper, just to become proficient in binary calculations. This is a good strategy for the CCNA exam where time is limited, But it often results in wrong calculations (atleast in the beginning).

     

    As Winston has suggested, if our calculations are correct there should be NO overlap of IP addresses.

     

    So take a break, recalculate all the stuff ( don't mind, reopening the copy and using a pen/pencil ), and If you are still getting the same result, please post the information about Network Addresses and subnet-masks that you are working upon.  Let's see whatz going on..

     

     

    P.S. While replying, I assumed that you are still unaware of the NAT and other technologies which you will study later on as you progress with the CCNA syllabus... So bookmark this thread, you might need to revisit it when you reach NAT..

  • Chetan 539 posts since
    Jul 26, 2008

    If a network address is subnetted, the first subnet obtained after subnetting the network address is called SUBNET-ZERO.

     

    Consider a Class B address, 172.16.0.0. By default the Class B address 172.16.0.0 has 16 bits reserved for representing the host portion, thus allowing 65534 (216-2) valid host addresses. If network 172.16.0.0/16 is subnetted by borrowing three bits from the host portion, eight (23) subnets are obtained. The table below is an example showing the subnets obtained by subnetting the address 172.16.0.0, the resulting subnet mask, the corresponding broadcast addresses, and the range of valid host addresses.

     

     

    Subnet Address

    Subnet Mask

    Broadcast Address

    Valid Host Range

    172.16.0.0

    255.255.224.0

    172.16.31.255

    172.16.0.1 to 172.16.31.254

    172.16.32.0

    255.255.224.0

    172.16.63.255

    172.16.32.1 to 172.16.63.254

    172.16.64.0

    255.255.224.0

    172.16.95.255

    172.16.64.1 to 172.16.95.254

    172.16.96.0

    255.255.224.0

    172.16.127.255

    172.16.96.1 to 172.16.127.254

    172.16.128.0

    255.255.224.0

    172.16.159.255

    172.16.128.1 to 172.16.159.254

    172.16.160.0

    255.255.224.0

    172.16.191.255

    172.16.160.1 to 172.16.191.254

    172.16.192.0

    255.255.224.0

    172.16.223.255

    172.16.192.1 to 172.16.223.254

    172.16.224.0

    255.255.224.0

    172.16.255.255

    172.16.224.1 to 172.16.255.254

     

     

    In the example above, the first subnet (subnet 172.16.0.0/19) is called SUBNET-ZERO.

    The class of the network subnetted and the number of subnets obtained after subnetting have no role in determining subnet zero. It is the first subnet obtained when subnetting the network address. Also, when you write the binary equivalent of the subnet zero address, all the subnet bits (bits 17, 18, and 19 in this case) are zeros. Subnet zero is also known as the all-zeros subnet.

     

    When a network address is subnetted, the last subnet obtained is called the ALL-ONES subnet.

    With reference to the example above, the last subnet obtained when subnetting network 172.16.0.0 (subnet 172.16.224.0/19) is called the all-ones subnet.

    The class of the network subnetted and the number of subnets obtained after subnetting have no role in determining the all-ones subnet. Also, when you write the binary equivalent of the subnet zero address, all the subnet bits (bits 17, 18, and 19 in this case) are ones, hence the name.

     

     

     

    Prior to Cisco IOS® Software Release 12.0, Cisco routers, by default, did not allow an IP address belonging to subnet zero to be configured on an interface. However, if a network engineer working with a Cisco IOS software release older than 12.0 finds it safe to use subnet zero, the ip subnet-zero command in the global configuration mode can be used to overcome this restriction. As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default, but if the network engineer feels that it is unsafe to use subnet zero, the no ip subnet-zero command can be used to restrict the use of subnet zero addresses.

  • Paul Stewart  -  CCIE Security, CCSI 6,989 posts since
    Jul 18, 2008

    I am in agreement that too much information can make things complicated.  What I do not want to do is give an absolute answer when there is more to it.  I try to qualify what I believe is beyond the scope of context as I did in this case (i.e. but would be well beyond the scope of which I think the question is being asked).  I agree there is a balance.  Instead of limiting the information though, I try to answer the question as I believe is pertinent to the scope first and then explain that the rest is probably not what Cisco is after with a CCNA examination.

  • Chetan 539 posts since
    Jul 26, 2008

    Hello Friends,

     

    I think CLN is a great community. Networking guys from all levels (beginner to much experienced) are here. So whenever a question is posted, its replies are not only read by the original person who asked the question but also by other guys/gals who are near the completion of their syllabus for the respective Certification or who are just checking how much they already know (just to learn more) or who already possess that much knowledge and experience and want to share their real-world experience about the technology involved which is in the periphery of original question.

     

    Whenever I ask any question, I use to put Certification-syllabus blueprint in front of me. So whenever I got a reply which involves any technology/concept that is CURRENTLY not in the syllabus, i just note it down as "Should See This after completing blueprint topics because they may be more relevant for the real-world job". It not only makes sure that I am not overloaded with the concepts but it also helps to broaden the THINKING process about what is/ what is not possible in this fast-paced technology world. So I think if I prepare according to certification blueprint AS WELL AS  knowledge of real-world networking, then it is good for my Certification as well as for the aspiring Job. In the end, I have to work in the Real-World (with or without certification...)

     

    Regards,

    Chetan

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