dhcp server returns core dump when i define network with mask 8

[Frank Leonhardt]

On 23/07/2013 09:45, s m wrote:
> On Tue, Jul 23, 2013 at 12:56 PM, Frank Leonhardt <frank2 at fjl.co.uk> wrote:
>
>> On 23/07/2013 09:03, jb wrote:
>>
>>> s m <sam.gh1986 <at> gmail.com> writes:
>>>
>>>   ...
>>>> subnet 192.0.0.0 netmask 255.0.0.0 {
>>>>       range 192.0.0.1 192.255.255.255;
>>>>
>>> The 'range' denotes IP addresses that can be allocated to clients.
>>> The IP 192.255.255.255 is a reserved broadcast address for the network.
>>> jb
>>>
>>>
>>>
>> It's definitely "bad idea" to try to use it, but it doesn't explain the
>> core dump.
>>
>> Also, using DHCP to dish out addresses that don't belong to you AND aren't
>> on a private network (as defined by IANA) will probably lead to trouble.
>> Valid private address ranges are:
>>
>> 10.0.0.0 - 10.255.255.255         (private class A)
>> 172.16.0.0 - 172.31.255.255     (private class B x 16)
>> 192.168.0.0 - 192.168.255.255 (private class C x 256)
>>
>> Which block you use is really a matter of taste - classes haven't been
>> used in routing for quite a while so you can consider them all as straight
>> blocks but I (for one) still treat them as classed just to help me
>> visualise what's what. For example, I'll use one class C per site to
>> prevent conflicts over VPN.
>>
>> 192.0.0.0/24 addresses are allocated to real hosts on the wider internet,
>> although IIRC some of the lower ones are reserved for use in documentation
>> (like example.com) - is that where the idea came from?!? :-)
>>
>> Regards, Frank.
>>
>>
> thanks Frank,
>
> 192 is just a sample. if i want to define 125.0.0.0 netmask 255.0.0.0, dhcp
> server core dump either. you're right, it is better to use just some
> limited addresses to avoid possible troubles. but i want to run my dhcp
> server for all possible networks.
> now my question is: if i define a network with mask 8, the rang should be
> like: 126.0.0.0  126.254.255.255?
>
> and thank you jb but if i define my network like below,  server runs
> correctly:
> log-facility local7;
> subnet 192.168.0.0 netmask 255.255.0.0 {
>      range 192.168.0.1 192.168.255.255;
> }
>
> i think 192.168.255.55 is reserved for broadcast too. is it not true? if
> yes, why dhcp server works correctly?
> please help me to clear my mind.
> regards,
> SAM
>

If you are connected to the Internet, using addresses like 125.0.0.0 
will cause trouble. You can ONLY use private addresses on local 
networks. If you are in a lab, and you are not connected to the 
Internet, it's okay. I am worried when you say "I want to use my DHCP 
server for all possible networks" - I do not understand what you mean 
but it sounds dangerous!

There are two common ways of defining a subnet mask - one is a dotted 
quad (e.g. 255.255.255.0) and the other is with a slash and the number 
of low-order bits - e.g. 192.168.1.0/8. Eight bits here means you get 
2^8 addresses (i.e. 256). Don't use the first and last address in the 
range - the first is "complicated" (the network address) and the last is 
for broadcast packets. This doesn't always hold true but you're unlikely 
to come across exceptions.

So, when you say you want to define a "network with mask 8" I don't 
really know what you mean from your example. Do you mean a /8?

192.168.1.0/8 = range 192.168.1.1    192.168.1.254 with a subnet mask of 
255.255.255.0 (0xFFFFFF00)

However, you don't normally put the whole range in the DHCP pool. For 
practical reasons you'll need a router or gateway in there somewhere on 
a fixed address, and by convention that either goes on .1 or .254.

Regards, Frank.