Cisco Router Interface Problem?

Hi I am having a little trouble assigning ip addresses to my routers ports. Let me first give some background information. I am doing a final project for my networking class. I have to build a networking scheme in Cisco Packet Tracer. I was given a private ip address of 172.30.140.0/21 and a public ip address provided by the ISP of 128.105.00.16/30.

I am currently in the process of making the networking scheme and setting it up in packet tracer. The requirements include 3 subnets. Subnet A needs 752 hosts, Subnet B needs 100, and Subnet C needs 463. I used VLSM to subnet to make 3 different subnets that will satisfy the needs (I think).

Now I am in the process of assigning the interfaces the ip addresses. I assigned the two computers 172.30.0.11 and 172.30.0.12 respectively (got from subnetting 172.30.140.0 /21). . But then when I go to assign the router 2 ip addresses for it's two fast ehternet spots, it says that they overlap with one another? I was trying to use 172.30.0.13 and 172.30.0.14 as I assumed they would be next in line. However that has not worked. Is this the part where I use the Public ip address? I am not sure though because I am not connecting to the internet, just the switches. Any help would be greatly appreciated as I have been working on this situation for hours!!

Thank You

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lincis_aus

So you have been given an IP subnet to subnet into 3 networks (i assume 172.30.140.0/21). Walk us through how you got 172.30.0.11/21 and .12 out of 172.30.140.0/21?

You are getting that error from the router because you cannot have two interfaces on the same router to be in the same subnet.

You need to do a bit more practice on your subnetting I think

devils_haircut

Like linicis_aus said, if you can show us the 3 subnets and their IP ranges that you have created for each network, we might be able to help you see where you went wrong.

EDIT: If you could also put a little more detail in your screenshot (like IPs for each port/device), that would help.

jdnagy259

Yes the private ip address is 172.30.140/21.

The 1st subnet requires 752 hosts, the 2nd requires 100, and the 3rd requires 463. I started with 172.30.0.0/21 and found that it has a magic number of 8. /21 also creates 1024 hosts which is enough to supply the largest subnet (752). Here is my subnetting work.

172.30.0.0 /21- 172.30.7.255/21

I then move to a /22 so that I won't waste as many hosts. /22 = 512 hosts > 463 hosts needed for the third subnet

172.30.8.0/22 - 172.30.11.255 /22

I then move to /24 for the last subnet.

172.30.12.0/24 - 172.30.12.255

The first ten address for subnet number #1 are reserved so that's why I assign the first pc an ip address of 172.30.0.11, because I can't use 172.30.0.1 , 172.30.0.2, 172.30.0.3...etc. Then I just went and assigned any other interface that needed an ip address. Well this is at least my way of thinking haha but I'm pretty sure this is not how all it works? Anyways, I appreciate any help that I can get.

jdnagy259

jdnagy259 wrote: »

Yes the private ip address is 172.30.140/21.

The 1st subnet requires 752 hosts, the 2nd requires 100, and the 3rd requires 463. I started with 172.30.0.0/21 and found that it has a magic number of 8. /21 also creates 1024 hosts which is enough to supply the largest subnet (752). Here is my subnetting work.

172.30.0.0 /21- 172.30.7.255/21

I then move to a /22 so that I won't waste as many hosts. /22 = 512 hosts > 463 hosts needed for the third subnet

172.30.8.0/22 - 172.30.11.255 /22

I then move to /24 for the last subnet.

172.30.12.0/24 - 172.30.12.255

The first ten address for subnet number #1 are reserved so that's why I assign the first pc an ip address of 172.30.0.11, because I can't use 172.30.0.1 , 172.30.0.2, 172.30.0.3...etc. Then I just went and assigned any other interface that needed an ip address. Well this is at least my way of thinking haha but I'm pretty sure this is not how all it works? Anyways, I appreciate any help that I can get.

EDIT: I think I found where I may have gone wrong, here are my new usable subnets for the address 172.30.0.0 /21

172.30.0.1-172.30.3.254 /22

172.30.4.1-172.30.5.254 /23

172.30.6.1-172.30.6.126 /25

However I still have the same problem with the two routers interfaces overlapping one another? This part I am still very confused on..

Thanks,