Another day another subnet eh?  At least you know that a few hours
spent here will last you many years to come because subnetting is
a skill you can take anywhere in the world with you.  When I had an
interview for Cisco back in 2001 they had two engineers grilling me
on subnetting for over an hour.  I just wrote out the Subnetting Cheat
Sheet and gave them the answers in a few seconds.  I think even they
were wondering how I answered them so quickly.

Ready for the next subnetting question?  Of course you are.  Write out
another subnetting cheat sheet and let's go.

Which subnet is host 172.16.1.44/18 in?

'Oh dear' I hear you sigh.  'Not a hard one. 'No it isn't and I'll show you why.

This is a traditional Class B address.  I do say on the Subnetting Secrets site that Classes of address don't really matter anymore which is true.  But just remember that for Class B addresses, the first two octets are reserved for the network address and there are no exceptions to that rule.  The last two octets can be used for all host numbers or subnet and host numbers.

If the address was 127.16.1.44 /16 which is the standard subnet mask for Class B addresses (255.255.0.0) then 172.16 would be the network and 1.44 would be the host.  My point is that two numbers can make up one host address.

Step 1 - turn the /18 into a subnet mask.  You know the drill by now, write it out in long hand.

255.255 is 16 bits and add 2 to get 18.  Tick two places down the Subnetting Secrets cheat sheet to get 192.  So your subnet mask is:

255.255.192.0

Which means we are subnetting and some of the host bits have been used
to create the subnets.

Step 2 - tick the same across the top of the Subnetting Secrets cheat sheet giving you subnet increments of 64.  Now here is the bit you have to be very aware of.  We are using two octets for creating the subnet and host numbers so I am going to start counting up but write two octet places to cover both.

Step 3 - write out the subnets in increments of 64

0.0 (our zero subnet)
64.0
128.0
192.0

So from the above numbers can you see which one host 1.44 is in?  It must surely be the zero subnet.  Our answer is that host 172.16.1.44 is in subnet 172.16.0.0

Step 4 - where on earth did the extra step come from?  In exams and on live networks you may well be asked to identify the first or last host on that subnet or in fact the broadcast address.  It is very easy though.

To get the first host just add 1 to the last octet.  We always start adding host numbers to the far right octet first.  To get the broadcast address look at the next subnet and subtract 1 and to get the last host number subtract 1 from that.  Like this:
 

And there you have it.  All the above numbers are the last two octets so you could put the 172.16 in front of them but it will just take you longer to write them out.

You get lots more detail on how to work this out and many more examples in the Subnetting Secrets e-books.

http://www.subnetting-secrets.com/buy_subnetting_secrets.html

Or it is given away to all members of http://www.howtonetwork.net which is a unique Cisco CCNA online training site I have created.

Until tomorrow.