[KLUG Members] some network questions

[Bert Bbbink]

>Bert Bbbink wrote:

>> The network/host part is set by the netmask, what basically means that the
>> last bit set (from left to right) marks the end of the network part. What
>> if I set my netmask like 255.254.255.0 ? A easily made typing error.
>> If again the last bit set from left to right marks the end of the network
>> part my network ends at the 15th bit.
>>
>Chris Hansen writes:

>Well, I think you mean the 16th bit (if it's the difference between 254 and
>255 in the 2nd octet that is).  All the positions of the netmask are useful
>and used by any OS with a full functional tcp/ip stack.  You usually only
>use the 4th octet on most networks because you're generally dealing with
>less than 254 usable IPs on your network.  What you want to look into Bert
>is "subnetting" to get a better understanding of the usefulness of the
>netmask.
255.254 is /15 I think.

>http://en.wikipedia.org/wiki/Ipv4_subnetting_reference

>As far as your compression question, I'm not real clear, but the RARP
>comment makes me think your running into subnet/routing probs.  Say you
>have a 192.168.1.0 network and a 192.168.2.0 network.. if you give a
>machine on the 192.168.1.0 network a 255.255.0.0 subnet mask it's not going
>to ask it's default gateway to help it get to 192.168.2.0's network.

In the example you mention network probes could indeed open the lines, the
routers don't see them selfs as a gateway towards an other network. Any
broadcast is transmitted.

I think I need to give some more details about what we have observed.
Suppose you have a host on a site 192.168.1.23/16 on an other site there
is an host on 192.168.2.67/24. The can ping each other with no problem. At
some moment compression on the routers is switched on the get more
bandwith.

Because the routers are installed and maintained by a third party we don't
have the exact details on what they are doing on the routers. Any way,
with the compression also rarp is turned on.
After some time a -huge- bill for isdn drops on our clients desk. That
does not make him a nicer guy than he was before...

Then wise well paid guys who turned rarp on decide to turn it of again,
that solves the bill but makes the 192.168.1.23/16 unavailable.
We look into it and find the netmask being wrong. Switching from /16 to
/24 solves this. Why?

The wise and well paid guys came up with a story to explain this that
definitly does not make any sence and is unrepeatable because of the many
twists and contradictions.

That's also where to question about 255.254.255 comes up. What if we make
a  mistake by typing the 254 where it should have been 255? Where in this
example does the network end and the host part begin?
And why does make compression some hosts unavailable and hides/solves rarp
this?

regards,

Bert.



_______________________________________________
Members mailing list
Members at kalamazoolinux.org