Re: netmasks

[Jeff <jcoppock1@attbi.com>]

Martin Clarke, 2002-Sep-21 14:33 +0000:
> a question i was asked recently on a topic that I thought
> i understood untill i was asked....
> 
> Given the hypothetical subnet 136.206.16.128 and netmask 255.255.255.128,
> state the valid range of IP addresses that could be assigned to hosts on
> the subnet?(also another was 64.122.34.83 & netmask 255.255.255.240  :(  )
> 
> i was used to looking at plain netmasks ie 255.255.255.0(ffffff00) etc 
> which I understand fine but my grasp of anything else is less than
> good...
> 
> any help is appreciated..
> 
> thanks
> 
> Martin.

136.206.16.128 255.255.255.128 (/25)
network 136.206.16.128
broadcast 136.206.16.255
hosts 136.206.16.129-254

* 64.122.34.83 is actually a host in the 64.122.34.80/28 network:
64.122.34.80 255.255.255.240 (/28)
network 64.122.34.80
broadcast 64.122.34.95
hosts 64.122.34.81-94

When you start working with netmasks where octets are something other
than a 0 or a 255, you're breaking the classful networks into smaller
subnets.  The 255.255.255.128 mask breaks a class C network into 2
networks.  Each time you break up a classful network, you burn 2
addresses in each subnet, 1 for the new network and 1 for the new
broadcast.  I common practice in large routed networks is to use a /30
subnet mask, 255.255.255.252, which gives you a network address, a
broadcast address and 2 host addresses which are used for the two
endpoints of the routed link.  This conserves the host addresses that
would never be used on the routed link.

I used to do these calculations by hand, until I found gipsc, which is
an IP calculator.  I suggest installed gipsc and playing around with
it.  You start to see how the classful networks changes when you break
into them.  Start with a Class C network, go the the CIDR tab and play
with the Subnet Bits.  You'll see the long form of the mask change.
Then go to the Subnets/Hosts tab and see the ranges.  The ranges shows
the network and broadcast addresses and everything in between are for
hosts.

I hope this helps,
jc


--
Jeff Coppock		Systems Engineer
Diggin' Debian		Admin and User