how to check if devices are on same network or not?

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Dec 10th, 2008
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i have one host IP

172.16.10.100/16 and another host

172.16.20.200/24


how can we check that if they are in same network or not?

many thanks

mahesh

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Jon Marshall Wed, 12/10/2008 - 13:10
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Mahesh


Not sure i fully understand.


172.16.10.100/16 is host 10.100 on network 172.16.


172.16.20.200/24 is a host 200 on network 172.16.20.


is there some connectivity issue you are having.


Jon

mahesh18 Wed, 12/10/2008 - 13:20
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Hi jon,

i want to know if these two devices are on same subnet or not?

if they are on different subnet how can we check that?

many thanks


Jon Marshall Wed, 12/10/2008 - 13:27
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Mahesh


No they are not on the same subnet - see my previous answer.


Jon

royalblues Wed, 12/10/2008 - 13:30
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Yes they are in the same subnet


The first ip address is part of the major subnet 172.16.0.0/16. 16 here would denote the network bits leaving the other 16 as host bits


The IP address 172.16.200.0/24 is a subset of the major subnet above


Time to read some ip addressing

http://www.ralphb.net/IPSubnet/


Narayan

Jon Marshall Wed, 12/10/2008 - 13:35
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Narayan, Edison


First time we have all disagreed.


Narayan - yes in same subnet

Edison - yes for one, no for the other

me - no for both.


How do we decide this :-)


Jon

Edison Ortiz Wed, 12/10/2008 - 13:44
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Host

172.16.20.200 255.255.255.0

needs a router to reach

172.16.10.100 255.255.0.0? (Yes|No)


Host

172.16.10.100 255.255.0.0

needs a router to reach

172.16.20.200 255.255.255.0 (Yes|No)



Based on the answer above, you determine which host sees the other host as belonging to its own subnet. This is a broken network to begin with but the question is valid and can be seen as tricky CCNA question :)


__


Edison.

Jon Marshall Wed, 12/10/2008 - 13:52
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I understand how it works in the respect of 172.16.10.100 255.255.0.0 thinks 172.16.20.200 is on the same network because of it's own subnet mask ie. 255.255 but that wasn't the question or at least it wasn't the way i interpreted it.


The question was is 172.16.10.100/16 on the same network as 172.16.20.200 and not does 172.16.10.100/16 think 172.16.20.200 is on the same subnet.


172.16.10.100/16 = network = 172.16

172.16.20.200/24 = network = 172.16.20


maybe it's a terminology thing in terms of network/subnet.


And maybe it's just me as i seem to be having these sort of dicsussions with a few fellow NetPro's and the one thing they have in common is me !


Perhaps i just need a break :-)


Jon

Edison Ortiz Wed, 12/10/2008 - 14:08
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The question was is 172.16.10.100/16 on the same network as 172.16.20.200/24


It is :)


The other way around, the host does not see the other host as its own subnet - needs a router :)




Jon Marshall Wed, 12/10/2008 - 14:13
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okay, you can put it in bold if you like :)


"The question was is 172.16.10.100/16 on the same network as 172.16.20.200/24"


It is on the same major network 172.16. but for the life of me i can't see how


172.16 = 172.16.200


From each addresses perspective i can see how 172.16.100.16 thinks 172.16.20.200 is the same network but i didn't think we were talking about perspective.


Forget it, it's me.


Jon




Edison Ortiz Wed, 12/10/2008 - 14:15
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Jon, my friend. Step away from the computer :)


Seriously, I know how you feel. I've felt burned a couple of times from answering questions in NetPro in addition to holding a full-time job. Take a week or two off. It helps.


We don't have to compete in the 'Top NetPro' of the month anymore :)




Jon Marshall Wed, 12/10/2008 - 19:47
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Edison


Ironically i'm not working at the moment but i take the point, i'm actually busier than when i do work !


Since i have nothing but the highest respect for you i think i'll take your advice and have a week or so off.


"We don't have to compete in the 'Top NetPro' of the month anymore :)"


That used to be fun tho didn't it ?.


Still don't agree with Cisco memebers not being allowed to be on the boards :-)


Thanks Edison.


Jon

royalblues Wed, 12/10/2008 - 13:45
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Best way is to put it to test


Router(config)#int fa 0/0

Router(config-if)#ip add

Router(config-if)#ip address 172.16.10.100 255.255.0.0

Router(config-if)#

Router(config-if)#no sh

Router(config-if)#

Router(config-if)#

Router(config-if)#int fa 0/1

*Mar 1 00:01:06.299: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state t

o up

*Mar 1 00:01:07.299: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthern

et0/0, changed state to up

Router(config-if)#

Router(config-if)#

Router(config-if)#ip add 172.16.20.200 255.255.255.0

% 172.16.20.0 overlaps with FastEthernet0/0

Router(config-if)#

Router(config-if)#


Narayan

Edison Ortiz Wed, 12/10/2008 - 13:48
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Narayan,


I think you are missing the point :)



royalblues Wed, 12/10/2008 - 13:52
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I agree


You were explaining more from a routing perspective whereas i was referring to the configuration on a single router :)


Narayan



Edison Ortiz Wed, 12/10/2008 - 14:05
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The OP never asked about configuration in a router.


That's the problem with us techies, we just think about if the router supports the commands and we end up failing on those tricky questions during the exam :)


I've been burned many times assuming just that and now read the questions differently.


I have a feeling the OP saw this question in a Cisco exam :)

__


Edison.

Edison Ortiz Wed, 12/10/2008 - 13:28
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The host 172.16.10.100 255.255.0.0 is on the same network as 172.16.20.200 255.255.255.0 but 172.16.20.200 255.255.255.0 is not in the same network as 172.16.10.100 255.255.0.0 - thus you will have connectivity issues between these devices as the path from one device to another may be different.


__


Edison.

royalblues Wed, 12/10/2008 - 13:40
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Edison,

I think you put it the reverse way

The address 172.16.20.200/24 is on the same network as 172.16.10.100/16



Narayan

mahesh18 Wed, 12/10/2008 - 13:41
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Hi ediortiz,

can u please explain me in more detail

many thanks

mahesh

viyuan700 Wed, 12/10/2008 - 14:26
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hi mahesh

your network

172.16.10.100/16

network 172.16.0.0

first host 172.16.0.1

Last host 172.16.255.254

broadcast address 172.16.255.255

next network 172.17.0.0


So your network 172.16.20.200/24 and 172.16.10.100/16 are host 172.16.0.0/16 network

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