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I have a project assignment that consists of 3 routers, the ISP, and 2 company routers, the ISP can address a network /24 IPv4.

I delegated a /25 to DNS servers and a /26 to one company and 2*/30 to the second company and the other goes to the VPCS that represents the "internet".

On the company server, I was trying to subnet the /26 into 3 networks for each department but looks like I'm doing something wrong.

F1/1 is connected to the ISP with an IPv4 of 212.199.4.130/26

I thought that I could set the other interfaces as

F0/1: 212.199.4.129/27
F0/0: 212.199.4.160/28
F0/1: 212.199.4.176/28

But I was wrong, if try, it tells me that there's an overlapping error.

Im not sure what I'm doing wrong here, any help would be appreciated

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  • Classroom scenarios are unfortunately off-topic.
    – TDurden
    Feb 3, 2023 at 17:39
  • I will add a single appliance cannot have an interface with a mask which overlaps with another interface. Another way to view this is- you cannot have subnet on an interface which overlaps with a subnet on another interface on the same device. Check your IP ranges.
    – TDurden
    Feb 3, 2023 at 17:41
  • @TDurden I think I understand it now, so I just need to divide the /24 network and assign it to each department according to the needs of each one, and I need to make sure that they don't overlap, by using /30 or /31 as pointed by Ron. Also, do you know an appropriate place to ask questions about this? Note that I'm not looking for anyone to solve my project, I just needed to ask a few questions, I'm allowed to research to complete this assignment, I'm just not allowed to copy from someone else, but that's not my intention Feb 3, 2023 at 18:08

2 Answers 2

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The interface on F1/1 needs to be a different subnet than the other interfaces. Try using a smaller subnet (only needs to be a /30 or /31).

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F1/1 is connected to the ISP with an IPv4 of 212.199.4.130/26

If the ISP expects all IP addresses from that /26 to be 'on link' then that's what it's going to be. You need to either put a switch in front of the router and connect the public-IP devices there (you'll need a transparent/L2 firewall in front of or besides the switch as well), or let your router respond to all those addresses and map the desired servers using destination/reverse NAT.

Normally you'd get a /30 or /31 address for the router to enable you to put the public subnet somewhere else.

I thought that I could set the other interfaces as

F0/1: 212.199.4.129/27
F0/0: 212.199.4.160/28
F0/1: 212.199.4.176/28

That doesn't work. As Ron has already pointed out, those subnets overlap with the one configured on the router's uplink. Using a smaller subnet for the uplink (just your ISP's default gateway and your router) may work but it won't if your ISP expects a normal /26 subnet 'on link'.

You should talk to your ISP on what options you've got or use the destination NAT method above.

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