Subnetting Question

Here's my situation. I'm using Cisco packet tracer to setup a lab for basic router setup with end clients. I'm used to 192.168.x.x... however the book I'm using is suggesting 172.16... I know it's a class B address... and when I try to type in the address I'm given a subnet mask of 255.255.0.0... but for example the book is telling me the network is 172.16.30.0/24. I'm thinking because of the /24 I should be using 255.255.255.0... even though the program auto types in the mask of 255.255.0.0 because of the address type... which one is right and why am I wrong?

I'm on page 72 of the CCNA portable command guide.

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Stuppored

I have changed my subnet mask to reflect the 255.255.0.0 but now I receive an overlap error as shown in the attachment. Can someone please explain why I am overlapping as I am under the belief I have two both interfaces on different subnets.

EDIT - I changed the fastethernet to 172.17.30.1 with subnet mask 255.255.0.0 and now I don't get the overlap error. I'm still confused why the overlap happened.... as well as the /24 question I had in the first post... can someone please explain what happened for both of these questions?

overlap.jpg

2.jpg

nbeacham

You are thinking in terms of classful addressing, yes 172.17.30.1 is a class B address, but they specified a subnet of /24 (255.255.255.0). This is also known as Variable Length Subnet Mask (VLSM). I urge you to do more research on this topic. You got the overlapping error probably because it fell within the other subnet.

Amjo

You are getting this error because your serial interface subnet will be part of the fastethernet network.. here is the explanation:
you are using 172.16.0.0/16 (mask 255.255.0.0) network (class

on fastethernet by using (172.16.30.1/16), this will give you a rang of ip addresses starting from (172.168.0.1 to 172.168.255.254) which the ip address on FastEthernet is part of it.. also the ip address that you are trying to assign to the Serial interface is part of this network too which is (172.16.20.2/30),, thats give you the overlap error>>> you cannot have two interfaces on the same network or subnets in a router<< it will never allow you to do that..

you need to have some more practices on subnetting..try Todd lammle CCNA book, he will explain the subnetting for you in very easy way..

thedrama

Stuppored wrote: »

Here's my situation. I'm using Cisco packet tracer to setup a lab for basic router setup with end clients. I'm used to 192.168.x.x... however the book I'm using is suggesting 172.16... I know it's a class B address... and when I try to type in the address I'm given a subnet mask of 255.255.0.0... but for example the book is telling me the network is 172.16.30.0/24. I'm thinking because of the /24 I should be using 255.255.255.0... even though the program auto types in the mask of 255.255.0.0 because of the address type... which one is right and why am I wrong?

I'm on page 72 of the CCNA portable command guide.

172.16.30.0/24 is a subnetted IP address. That means you should start from /24 then find how it was subnetted.