[CentOS] IP range

Thu Aug 2 14:52:02 UTC 2012
Johnny Hughes <johnny at centos.org>

On 08/02/2012 09:33 AM, Birta Levente wrote:
> On 02/08/2012 17:24, Johnny Hughes wrote:
>> On 08/02/2012 09:00 AM, Birta Levente wrote:
>>> Hi all
>>>
>>> Can someone explain me this:
>>>
>>> ifcfg-eth0-range1:
>>> ONBOOT="yes"
>>> IPADDR_START="192.168.1.48"
>>> IPADDR_END="192.168.1.55"
>>> CLONENUM_START="1"
>>>
>>> Why Bcast is 192.168.1.51 and why Mask is 255.255.255.252 ?
>>>
>>> OS: Centos 6.3/64bit
>>>
>>>
>>> Thanks
>>>
>>> Levi
>>>
>>> # ifconfig
>>>
>>> eth0:1    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.48  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:2    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.49  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:3    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.50  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:4    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.51  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:5    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.52  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:6    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.53  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:7    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.54  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>>>
>>> eth0:8    Link encap:Ethernet  HWaddr 00:9C:02:99:FA:00
>>>             inet addr:192.168.1.55  Bcast:192.168.1.51  Mask:255.255.255.252
>>>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>>>             Interrupt:16 Memory:fbee0000-fbf00000
>> That is obviously not going to work ... a 255.255.255.252 mask is a 4 IP
>> subnet, with only 2  usable addresses and a network number and a
>> broadcast address.  The only free addresses in
>> 192.168.1.48/255.255.255.252 are .49 and .50
>>
>> The Broadcast and Mask settings are likely in your ifcfg-eth0 file and
>> not in the range file at all.
>>
>> The Mask would either be set manually in ifcfg-eth0 ... or by the DHCP
>> server if you get DHCP. The Broadcast address would automatically be set
>> based on the Mask, unless it is overridden in ifcfg-eth0.
>>
>> If the address is set via DHCP, you need to change the subnet mask on
>> the DHCP server as that is where it comes from.
>>
>> If you want 8 usable addresses (including the Network number, a gateway
>> address, and a Broadcast address), that would mean you need at least 11
>> IPs in that subnet ... the closest fit would be a 255.255.255.240 subnet
>> (which has 16 addresses).  If you were to want to use th 255.255.255.240
>> subnet, then 192.168.1.48 would not be available as it would the the
>> Network number for that subnet ... the usable addresses would be
>> 192.168.1.49-63 that case and the Broadcast Address would be 192.168.1.64
>>
>> Since this is on a private network, why are you not just using the full
>> 192.168.1.0 network with a 255.255.255.0 subnet?
>>
>> I guess the real question is, what are you trying to do :D
>>
>>
>>
>>
>>
>>
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>> CentOS at centos.org
>> http://lists.centos.org/mailman/listinfo/centos
>>
> I have eth0 with public IP, netmask is 255.255.255.252.
> Additionally I own 8 public IPs xxx.xxx.xxx.48-55 with class subnet mask 
> 255.255.255.248
>
> If I set up with ifconfig eth:(0 to 7) xxx.xxx.xxx.(48-55) netmask 
> 255.255.255.255 it's work.
>
> But if I set up the ifcfg-eth0:(0-7) files with the same IP and netmask 
> it's not work. ifconfig show me the 255.255.255.252 netmask even if in 
> file other netmask is specified.
> The same situation in ifcfg-eth0-range1 case.

What if you do this:

ifcfg-eth0-range1:
ONBOOT="yes"
IPADDR_START="192.168.1.49"
IPADDR_END="192.168.1.54"
CLONENUM_START="1"
BROADCAST=192.168.1.55
NETMASK=255.255.255.248
NETWORK=192.168.1.48

(with a .248 subnet, you can not use the first address (192.168.1.48) or
the last address (192.168.1.55) on a device, they are the Network
Address and the Broadcast Address ... so an 8 IP subnet has 6 usable
addresses.  Also, one of those 6 addresses will also need to be assigned
to the gateway router if you need to talk to another network, so you
really only have 5 addresses that you can assign for use).


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