Analysing Network Diagrams ..

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May 17th, 2008
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I am studying for CCNA and searching Cisco sites for information and material I seeing more & more frequently Network Diagrams that instead of having an IP address at the output there is a number in decimal notation on one or both sides of links.

an example is here on this page

and in particular this image

which I have attached ..

Router R1 on S1 has .1 next to it.

Router R3 on S1 has .3 next to it.

Router R3 on e0 has .3 next to it.

Router R4 on e0 has .4 next to it.

I have seen instances in other diagrams where the decimal number might be associated with an ip address on the subnet for a link .. but this idea is not supported by the example I have given .. or am I wrong?

thanks for helping me .. I just want to get my head around this convention and possibly even find some documentation to read that will explain it.

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Joseph W. Doherty Sat, 05/17/2008 - 02:15
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What your looking at is the IP address for the interface but with only the last octet shown to reduce diagram clutter. The subnet address is shown for the segment.

In the diagram, see under the LAN segment, so R3's e0 .3 is and R4's E0 is Likewise we see under the WAN link the subnet address, so R3's s1 .3 is and R1's s1 .1 is

You can see confirmation of the above by looking at the router configuration within your first attachment. For example, from Router 3's configuration box:

interface Ethernet0

ip address

interface Serial1

ip address

Done_Fishin Sat, 05/17/2008 - 15:01
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Thanks for the prompt reply .. I did in fact look deeper after posting this and noted that I had misread those ip's , by which time it was too late. I managed, with the help of a forum that I visit, to deduce that it was in fact the last octet of the ip address but it is refreshing to have my assumption confirmed by a professional.

I haven't seen anything written down anywhere about this possible change in diagram annotation , it certainly wasn't mentioned on either of the ICND courses I have been on. In fact initially, I confused it with Routing Protocol Cost .. and I have spent many hours trying to trace what it could possibly be, searching through all the different Protocols to see what might be the formula that would give such strange costs..

There is nothing mentioned anywhere (within cisco or outside) which also made it extremely frustrating trying to understand / analyse diagrams.

Thanks again for your help


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