Re: [webmin-l] DHCP Fails to start

[Kris D. <kd...@vi...>]

Jeff wrote:
> Here is what I am getting when I try to start dhcp

> /etc/dhcp3/dhcpd.conf line 110: subnet 192.168.1.1 netmask
> 255.255.255.0: bad subnet number/mask combination.
> subnet 192.168.1.1 netmask 255.255.255.0
>                                         ^
> Configuration file errors encountered -- exiting

Strictly speaking, a CIDR netblock starting on 192.168.1.1 cannot have a 
netmask of 255.255.255.0, it can only have a mask of 255.255.255.255.

This is determined by the positions of 1's in the binary expansion of 
the IP and netmask:

192       168       1         1
11000000  10101000  00000001  00000001
255       255       255       255
11111111  11111111  11111111  11111111

The network IP may not have 1 bits to the right of any 1 bits in the 
netmask - note that 192.168.1.1 has a 1 in the farthest bit to the 
right, so the mask must be all 1's down to that position as well.

A block starting on .2 could have a mask of .254, .4 could have .252, etc:

192       168       1         2
11000000  10101000  00000001  00000010
255       255       255       254
11111111  11111111  11111111  11111110

192       168       1         4
11000000  10101000  00000001  00000100
255       255       255       254
11111111  11111111  11111111  11111100

So for a 255.255.255.0 mask, your subnet must be .0.

-kgd