Mailinglist Archive: opensuse-edu (132 mails)

[Michael Brown <mbrown@xxxxxxxxxxxxxxxx>]

On Tue, 25 Jun 2002, Steve Palmer wrote:
> the 255.255.252.0 subnet was designated by RM when
> they installed the network some years ago. The other
> one is designated by EMBC who provide the internet
> access - they gave us a subnet mask range of
> 255.255.25** **= 2-5 with a range of ips from
> 10.0.40.2-254 with the intention of us switching over
> to their network and having the computers directly
> hooked in - fat chance! that gives us very little
> control thus we are using a proxy..

I think you may have misread the information they sent you (or, equally
possibly, they miswrote it!).  Firstly, only 255.255.255.252,
255.255.255.254 and 255.255.255.255 are valid subnet masks within that
range.  Secondly you cannot have a "subnet mask range" on a network.

A very brief explanation of subnet masks:

All machines on the same network must have the same network address.  The
network address is obtained by binary ANDing the IP address with the
subnet mask.  Thus, an IP address 192.168.3.200 with a subnet mask of
255.255.252.0 has a network address of 192.168.0.0:

192.168.3.200 = 11000000.10101000.00000011.11001000
255.255.252.0 = 11111111.11111111.11111100.00000000
192.168.0.0   = 11000000.10101000.00000000.00000000

Similarly, 192.168.2.47/255.255.252.0 would have a network address of
192.168.0.0:

192.168.2.47  = 11000000.10101000.00000010.00101111
255.255.252.0 = 11111111.11111111.11111100.00000000
192.168.0.0   = 11000000.10101000.00000000.00000000

and so would be part of the same network.

The broadcast address is (conventionally) obtained by binary ORing the
network address with the inverse of the subnet mask; equivalently ORing an
IP address with the inverse of the subnet mask.  Hence the broadcast
address for 192.168.0.0/255.255.252.0 is 192.168.3.255:

    192.168.0.0   = 11000000.10101000.00000000.00000000
NOT 255.255.252.2 = 00000000.00000000.00000011.11111111
    192.168.3.255 = 11000000.10101000.00000011.11111111

You can see from this that in order for all hosts on a network to have the
same network and broadcast addresses, they must at least all have the same
subnet mask.

If the range of IP addresses they gave you was, as you say,
10.0.40.2-10.0.40.254 then this would *suggest* a "classical" subnet mask
of 255.255.255.0, with 10.0.40.1 being the default gateway.  No guarantee,
but that would be my initial assumption.

Michael