This might be a stupid question but I'm new to this. I am studying a class about basics of networking and I don't really understand one question that we have.

We need to a divide the given IPv4 address range into subnets.

Now, we have this formation:

R1  R3
| \/ |
| /\ |
R2  R4
|    |
S1   S2--S3
|     |   |
PCs  PCs  PCs

I know that we will have a subnet under R2 and R4. Giving us 2 subnets, that makes sense, since we need to give an IP address to every PC and it should have some order. And the switches don't use IP addresses to send frames from what I have understood but they use MAC addresses that they build.

From what I have understood we will also make a new subnet for the different connections between the routers too? Why is that?

Since thinking about it R1 and R3 are connected to the one router R2. Why would we make 2 different subnets for R2 even though it is just one router. Having 2 different subnets just seems like a waste of space for me. I guess it will be necessary when R2 is sending to R3 or R1 I guess? I might have answered my own question...

Now, I might have understood this wrongly since the assistant professor didn't mention it... so a friend of mine told me that this is how it apparently works and he just knew it with no logic to explain why. I might not have really made the question that clear since I'm quite confused myself lol. Though I have tried to make it clear.

Thank you!

  • Yes, routers route packets between networks, so each router interface is in a different network. (There are such things as unnumbered interfaces, but do not get confused by that until you get the basics.)
    – Ron Maupin
    Jan 27, 2021 at 23:36
  • @RonMaupin Aha, so basically in layman terms every port with an Ethernet cable gets a new Address within the router? Or... not exactly every port but the link "itself"?
    – Cewu00
    Jan 27, 2021 at 23:39
  • 1
    Remember that ethernet is not the only link-layer protocol or connection used by routers, There are also others, such as PPP, HDLC, frame relay, ATM, etc. that are sometimes used for WAN links. Ethernet is usually used on a LAN, but not always on a WAN. There are also often virtual interfaces you may configure, such as loopbacks, tunnels, subinterfaces for VLANs, etc., and each gets an address in a different network.
    – Ron Maupin
    Jan 27, 2021 at 23:46
  • @RonMaupin Yup I will, I get it, Thank you! :)
    – Cewu00
    Jan 27, 2021 at 23:48

1 Answer 1


With rare exceptions, Yes. Routers route between networks, so every interface is connected to a separate network. So each interface has an address on the individual network (subnet).

  • ... unless you're using unnumbered, point-to-point interfaces. ;-)
    – Zac67
    Jan 28, 2021 at 8:16
  • 1
    That's the 'rare' exception :)
    – Ron Trunk
    Jan 28, 2021 at 17:30

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