Re: WTH is up with this network config?

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On Wed, 2010-04-28 at 16:15 -0700, jack craig wrote:
> When I learned about ip address, the network & host portions of the IP
> and the netmask to differentiate the host & network numbers. I learned
> a class A addr used a mask of 10.255.255.255.

That number sequence is not correct.  The old class A scheme could use
addresses from 10.0.0.0 to 10.255.255.255, and the netmask for all of
them being on the same subnet would be 255.0.0.0.
> 
> eth0      Link encap:Ethernet  HWaddr 00:26:18:FE:75:91  
>           inet addr:10.0.0.100  Bcast:11.255.255.255  Mask:254.0.0.0

Your broadcast and mask are odd, too.

Try again...

The netmask is an mask against where the boundary lay.  For an
addressing scheme like the 192.168.x.y scheme, the netmask could be
255.255.255.0 or 255.255.0.0. (it's usually the latter).  

The 255 portions go with the common portions of your addresses
(everything that's the same on this side of your IP address is
considered to be on the same subnet; and when an address is different,
here, it's considered to be external, and you must go through the
gateway to access it), and the zero portions of the netmask relate to
the variable portions of your own internal IP addresses.

The reason the third quad could be 255 or 0 is down to whether you're
using all of the possible addresses in that scheme, or just some of them
(i.e. where I've written "x" will be a fixed value), in a particular
sub-network.

For the 10.x.y.z private networking address scheme, you have three quads
that you can use.  And the same thing applies, if all of those addresses
were on a common subnet, you'd probably use a netmask of 255.0.0.0, as
that's the usual default mask for class A (all addresses start with 10,
and all of the last three quads are available as internal addresses on
the same subnet).  But, if you were carving a big LAN up into separate
networks, you might use a different mask.  e.g. Building one might use
10.1.0.0 to 10.1.0.255 and building two might use 10.2.0.0 to
10.2.0.255, and you could use a netmask of 255.255.0.0 to keep them more
isolated from each other.  Or, more to the point, to make traffic go
through a gateway so it could access the other subnet.

On that note, the broadcast address will begin with the common prefix
quads for that network (10.0... or 192.168...) and end with 255 as the
last quad.  So your broadcast address would be 10.0.0.255 not
11.255.255.255.

If you're going to do some more unusual network addressing, you need to
understand converting decimal to binary, and vice versa, or use an
address & netmask calculator to work things out for you.

-- 
[tim@localhost ~]$ uname -r
2.6.27.25-78.2.56.fc9.i686

Don't send private replies to my address, the mailbox is ignored.  I
read messages from the public lists.



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