/23 network

[chrisgerke]

Just in the process of updating documentation and I need to let some vendors know a local subnet has changed.

We previously had 161.230.247.0 255.255.255.192 but have since changed the mask to 255.255.254.0

Would I tell them its 161.230.246.0/23 or 161.230.247.0/23 because they are the same thing correct? I guess its just "best practise" to use the numerical lowest network number ?

[Jon Marshall]

Chris

You usually use the network address and the /notation so it should be 161.230.246.0/23.

Jon

[chrisgerke]

So even though both 161.230.246.0/23 and 161.230.247.7/23 both include IP addresses in the range 161.230.246.1-161.230.247.254 its the correct way to describe it by using the numerically lowest?

Even though technically you could use either in a static route etc?

[Jon Marshall]

Chris

To be honest i have never used anything other than the actual network/mask in a static route so i don't know what would happen as i don't have anything to test with.

From memory though if you tried to use it in an acl IOS will correct it for you and change to the network/mask although i could be misrembering.

The actual network is 161.230.246.0 not 161.230.247.x because technically that is just an IP from the range.

Jon

[chrisgerke]

Thanks, your last paragraph is the bit I'm trying to understand.

Because the mask makes the range extend across 246 and 247 why is it that the "network" is 246? Because its the numerically lowest in the range?

[Jon Marshall · Accepted solution]

Chris

Basically yes, it is because the lowest in the range which is simply another way of saying -

if you wrote out 161.230.247.x in binary and you did the same with the subnet mask 255.255.254.0 you would see that the network part of that address is 161.230.246.0.

How you have broken that down is up to you ie. you could get 2 class C networks, you could get 4 half class C networks etc.

There is no way of knowing but what you can do is find the network portion of the address with the subnet mask. If you put in your documentation 161.230.247.0/23 then someone simply has to do a bit of maths in their head to work out the actual network part whereas if you put 161.230.246.0/23 then that is the network address and subnet mask

I hope that explains it although i can't help feeling i'm not getting to the core point of your question so please come back if you have further questions or i haven't been clear enough.

Jon

[chrisgerke]

Thanks.

"if you wrote out 161.230.247.x in binary"

That's what I was looking for. I've re-read the cisco documentation now and it makes sense. Write it out in binary to get the network.

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Would I tell them its 161.230.246.0/23 or 161.230.247.0/23 because they are the same thing correct? I guess its just "best practise" to use the numerical lowest network number ?

No, they're not really the same.  161.230.247.0/23 is an IP within 161.230.247.0/23 address block, as is 161.230.246.255/23.  161.230.247.255/23 is the subnet broadcast address for 161.230.246.0/23.  Technically, I believe, 161.230.246.0/23 is both the network and an IP for that network, but generally the IP is avoided, as an IP, as to not be confused with the network itself.

[chandra_rc16 · Accepted solution]

Hi Chris,

--------

We previously had 161.230.247.0 255.255.255.192 but have since changed the mask to 255.255.254.0

Would   I tell them its 161.230.246.0/23 or 161.230.247.0/23 because they are   the same thing correct? I guess its just "best practise" to use the   numerical lowest network number ?

-------

It is not about saying  the "best practise" to use the numerical lowest network number.

Previously you told them: 161.230.247.0/26-> Becuase this is the N/w address for the IP range b/w

161.230.247.0  to 161.230.247.63 (As you might already know that the first one will be  the N/w address and last one will be broadcast address in b/w them are  ths host configurable IP's).

Now if you take any of the  IP address b/w this range and the subnet mask in binary, then perfrom  AND operation then you'll get the n/w address only, that is the reason  we tell the people only the n/w address. check it out here:

Let's take the IP: 161.230.247.3/26

IP in binary: 10100001.11100110.11110111.00000011

Mask     :      11111111. 11111111. 11111111. 11000000     Performing AND operation

----------------------------------------------------------------------------

                    10100001.11100110.11110111.00000000 = 161.230.247.0

This  is how the routing devices check and route the packet. So that is the  reason why people tell the N/w address to the routing devices.

##########

-------------

Would I tell them its 161.230.246.0/23 or 161.230.247.0/23 because they are the same thing correct?

-------------

Basically this is a class B network which  contain 16 N/w bits, you've subnetted it to /23 (Nothing but you  borrowed 7 bits from the host portion). Now you've subnetted it to 128  (2power7) networks each containg  512IP address(host portion has 9 bits  so 2power9), which says for every 512 IP address you network number  changes.

Now if you take 161.230.247.0/23 and perform the AND operation with the mask, you'll get the N/w address. Let's do it.

161.230.247.0 in binary: 10100001.11100110.11110111.00000000

Mask     :                         11111111. 11111111. 11111110. 00000000     ANDING

-----------------------------------------------------------------------------------------------

                                         10100001.11100110.11110110.00000000 = 161.230.246.0 is the N/w address  for  range:161.230.246.0 to 161.230.247.255.

So now i think now you've clearly understood that 161.230.247.0 is the host configurable IP address not the n/w address.

Don't forget to rate the helpful posts.

Regards,

Chandu

[chrisgerke]

Thanks, I forgot about converting out to binary.