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Thread: NAT and Broadcast/Network Addresses

  1. #1
    TiG
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    Question NAT and Broadcast/Network Addresses

    Right this from what I can see is an expert thread. If you don't know the information in your head as standard I'd prefer you didn't answer, I'm Stuck at work with this problem and its causing me great hassle. I'm NO network engineer - i'm a developer but I need to get something organised and i'd appreciate some help on this...

    I've got a setup here with 2 kilostreams with Cisco Routers at the end of each line, (4 in total) Its on a small /28 subnet
    which gives me addresses from 193.132.46.32 to 193.132.46.47
    Using the subnet calculator this tells me the network address is 193.132.46.32 and the broadcast address is 193.132.46.47. Now I'm being told by the company the other side that we can use the broadcast address to put an application on, as we are using 1:1 Nat.

    All of the above I understand, and I think I understand that the broadcast address is if you want to speak to all the subnet?. I can't imagine any reason we would want to do this. So can you run subnets without broadcast addresses?, or will this screw stuff up, because i'm between internal people saying NO and external people (customers) saying yes.

    I've got to get this sorted ASAP as its going to cause me nightmares otherwise. Level of experience here is up to setting up our Dev cisco 515E pix. So i'm familiar with acl's etc.

    However my knowledge here is lacking any help is greatly appreciated.

    Thanks in advance
    TiG
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  2. #2
    You are feeling sleepy... acidrainy's Avatar
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    You never have to utilise the broadcast.
    It can be kept redundant if you wish.

    I'm not sure if this is the answer your after but its certainly the case..

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    The config you give in terms of IP sounds a bit suspect to me, or do you actually work for BT?

    Do you need bridging? In very simplistic terms, routers don't bridge...

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    TiG
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    No i don't work for BT. No i don't need Bridging and No Acidrainy that wasn't helpful.

    I want to do the exact opposite, I want to use it for an app.

    Thanks
    TiG
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    Administrator Moby-Dick's Avatar
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    Sounds like you have 13 Routable IP's there.

    all the times Ihave had similar multiple IP setups you can't use the 1st or last IP's
    how many boxes are you wanting to put on for this APP ( I presume 13 ? )
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    Quote Originally Posted by TiG
    No Acidrainy that wasn't helpful.

    TiG
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    TiG
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    Thats even less useful. Its true it wasn't helpful, I appreciate AcidRainy for trying to help though, I DISPISE you for spamming what is an important post!.

    TiG
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    Ex-MSFT Paul Adams's Avatar
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    As far as I am aware, any subnet (as you said) has a "network address" at the bottom end, and the top address is the broadcast address used to talk to all the clients in that subnet.

    That said, Windows (for example) does let you try to add the top address in a subnet to a NIC, so it might work, but I would most definitely never use the broadcast address for any host. Ever.

    When talking with people on subnets, they will almost always say that for any given mask, you have the following number of "useable" IP addresses:
    2^(32-mask length) - 2
    The 2 being subtracted are for the network and broadcast.

    i.e. you can't have more than 1 host in a subnet with a mask >30 bits, and at exactly 30 bits you can have only 2 hosts, even though 2^2 is 4.

    On another note, firewalls may very well see that IP address as a broadcast and not route it between subnets, thus causing problems.

    That help any, or just confuse matters more?
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    Quote Originally Posted by TiG

    So can you run subnets without broadcast addresses?, or will this screw stuff up, because i'm between internal people saying NO and external people (customers) saying yes.

    TiG

    Yes is the answer you require

    plain and simple

  10. #10
    TiG
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    Care to expand on that flash?, Yes you can run subnets using the broadcast address?, if so you got any examples or anything to help shed some light on this subject for me?.

    Thanks
    TiG
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    Boooooom Barakka's Avatar
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    Don't see how you can run a subnet without a broadcast address as such. It's not something you choose, it's where the host ID translates to all 1's in binary (1111 as the last octet in your case) which targets all machines in the subnet. Maybe if you were using a Class A, B, or C network, but you're not it's a custom subnet.

    Not sure what you're trying to achieve with it, what do you mean by running an application on the broadcast address ? You see it changes when routers are brought into it as broadcast traffic is normally not allowed to pass thorough a router, so maybe them saying you can use it has some relevance to this....

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    Quote Originally Posted by TiG
    Care to expand on that flash?, Yes you can run subnets using the broadcast address?, if so you got any examples or anything to help shed some light on this subject for me?.

    Thanks
    TiG

    show me the $$$

    theres no such thing as a free lunch these days

    especially when this results in you getting a tech in and get hit in the pocket

  13. #13
    TiG
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    It doesn't matter now anyways, I've worked it out. I would have posted the results on here if it wasn't for flash's last comment, anyone who is interested in the actual answer can PM me if they are interested.

    Thanks
    TiG
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  14. #14
    Oh no!I've re-dorkalated! Jiff Lemon's Avatar
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    Quote Originally Posted by Flash
    show me the $$$

    theres no such thing as a free lunch these days

    especially when this results in you getting a tech in and get hit in the pocket
    I REALLY hope this was posted in Jest.....

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