I thought I had subnetting basics pretty well under control, until I had a conversation with a few fellow students. Now I am a bit confused.
If I were to create the network 192.168.1.0/24 and then use 4 routers to segment and subnet this network, can I "reuse" network addresses by simply placing them in different subnets? If subnet A, B, C, and D all had 250 hosts on them could I reuse the network 192.168.1.0 on all of these subnets simply by changing the subnet mask on each subnet? A /23, B /22 etc...
This just doesnt seem right to me, I thought that each network address should be unique within the network as a whole not the subnet?
I would appreciate any insight you could give me on this.
To (hopefully) add to Rick's post, you may want to search around (Cisco main site and Google) using "Variable Length Subnet Masking (or Mask)" (VLSM).
Each address must be unique within the realm in which is is visible.
In order to use the same address in multiple places, Network Address Translation (NAT) is used to "hide" the duplicate addresses; it's hard to imagine how many places 192.168.1.0/24 is being used across the entire Internet ... but for each of those 192.168.x.x networks, they are behind a router or firewall that is doing NAT and presenting a single (or group of) unique addresses to the rest of the Internet.
Otherwise, there'd be no way to tell which 192.168.x.x network (of tens of millions 192.168.x.x networks) to route the traffic to.
... and, FWIW, keep in mind that just because you --CAN-- do something, it doesn't necessarily mean that you --SHOULD-- or that it's even a good idea (i.e., "best practice").
Either those other students were making some assumptions that you did not mention here or they are mistaken and incorrect.
It is true that it is possible (and possibly correct) for routerA to advertise 192.168.0.0/24 and for routerB to advertise 192.168.0.0/23 and for routerC to advertise 192.168.0.0/22. This would be correct if routerA wants to advertise addresses from 192.168.0.0 through 192.168.0.255 and if routerB wants to advertise addresses from 192.168.0.0 through 192.168.1.255 and if routerC wants to advertise addresses from 192.168.0.0 through 192.168.3.255. Those are 3 unique prefixes and each could be accepted and put into the routing table so that a show ip route would show all 3 subnets.
But you are correct that it is not valid for the same host address to appear in more than one place. If address 192.168.0.6 were to exist on routerA and on either routerB or routerC then it would be a problem and all traffic for 192.168.0.6 would be routed to routerA, since in routing the longest prefix wins.
When you use that kind of overlapping prefixes, each site needs to be very careful that they do not assign host addresses that overlap with another site.