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Thread: Help, My Subnet /8 doesn't works

  1. #1
    Join Date
    Feb 2023
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    Question Help, My Subnet /8 doesn't works

    I'm going to Computering Class anf We're learning about Servers and how they work. I'm learning about DHCP Server, but It's very difficult for me.
    I was trying to practice in my Home and I tried to configure a Group in a Type A IP (16.32.64.0/8), but for some reasons it show me this:

    subnet 16.32.64.0 netmask 255.0.0.0 bad subnet number/mask combination.
    subnet 16.32.64.0 netmask 255.0.0.0
    ^


    This is how I configurated the dhcpd.conf group:

    group aquila-legion {
    subnet 16.32.64.0 netmask 255.0.0.0 {
    range 16.32.64.16 16.32.64.128;
    option domain-name-servers 8.8.8.8;
    option routers 16.32.64.1;
    default-lease-time 3600;
    max-lease-time 86400;
    }
    }


    Repeat, I'm studying this and I'm very awful because it's very difficult.
    Anyone know why this happens?

  2. #2
    Join Date
    Mar 2010
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    Ubuntu Development Release

    Re: Help, My Subnet /8 doesn't works

    Network 16.32.64.0 would be /24... or netmask 255.255.255.0... Just saying.

    Do you have access to Cisco Packet Tracer? That would help you greatly in simulating networking topologies and subnetting in your studies.

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  3. #3
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    Re: Help, My Subnet /8 doesn't works

    subnet 16.32.64.0 netmask 255.0.0.0 is indeed invalid - you are effectively stating that the network number is that of a particular host within that network.
    When you configure the network number, you should give only the network number, with all the host bits set to 0. So in the case of a Class A (/8), you would quote subnet 16.0.0.0/8, or subnet 16.0.0.0 netmask 255.0.0.0.
    I'm not certain you will be able to persuade a DHCP server to be happy managing that many hosts, but that's a different issue. Your immediate issue is that the network number cannot be a host inside that network - you must zero out the host part.

  4. #4
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    Re: Help, My Subnet /8 doesn't works

    First, I'd always use the official private network ranges for testing. For Class A addresses, use 10.0.0.0/8.

    However it's unlikely you'd want to maintain a network this size. I typically use either private Class C addresses like 192.168.1.0/24, which supports 253 addresses, or private Class Bs like 172.168.0.0/16 which support 65,534.

    Using 10.0.0.0/8 and 255.0.0.0 means you're allocating addresses to 2^24 hosts/networks, or nearly 17 million addresses. Hardly anyone outside of large providers manage networks of that size.

    You need to start by assigning the server a "static" address in the range you're using on the interface that faces the local private network. A common numbering scheme with a Class-C range is to assign the outbound router, or "default gateway," an address like 192.168.1.1, and all of the other hosts with addresses between 192.168.1.2 and 192.168.1.254. Often the DHCP server is located on the router.

    On class-C networks the "network address," 192.168.1.0, and the "broadcast address," 192.168.1.255, are reserved and cannot be assigned by DHCP. Similar rules apply to class-B and class-C networks.

    With a class-C numbering scheme you might use
    Code:
    group aquila-legion {
    subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.1.51 192.168.1.200;
    option domain-name-servers 8.8.8.8;
    option routers 192.168.1.1;
    default-lease-time 3600;
    max-lease-time 86400;
    }
    }
    This gives DHCP clients addresses in the range 192.168.1.51-200, and tells the clients to send all outbound traffic to the default gateway, 192.168.1.1. Usually that's a router configured to send traffic upstream, typically to the public Internet.
    Last edited by SeijiSensei; February 13th, 2023 at 08:45 PM.
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