Ip overlapping with gigabit-Ethernet 0/0 Help?

[ripnlipsco]

So I did look around and read alot on this but im wondering if I am wrong or if my teacher is wrong. I was asked to subnet and configure a networking containing 2 computers 2 switches and 2 routers, So on each side(this is in packet tracer) it would be computer to switch to router, the repeat for the second one. One network would have 50 computers the other would have 10. So i did that using the ip he provided 172.16.0.0. First network 172.16.0.0-63 (i left out the network and broadcast in my packet tracer). The next network with 10 computers was 172.16.0.64-79 again i left out network and broadcast. Now i come to the Router to Router serial connection using a HWIC-T2. That needs a network! Note: subnet mask for both existing networks at this point is 255.255.0.0. Meaning for both computers both switches and both routers. So then i subneted a network with 4 ip addresses, that was 172.16.0.80-83 leaving me 2 addresses after network and broadcast. I go to interface serial 0/0/0 and type ip address 172.16.0.80 255.255.0.0 that didn't work it said 172.16.0.0 overlaps with gigabitethernet 0/0. I tried everything, changing the subnet mask to something. Im not sure what to do but i was told by my teacher that i would be able to subnet this whole network. Any ideas if im wrong or if he is?

[Iulian Vaideanu · Accepted solution]

Subnetting also means using proper masks, not just considering an IP range a subnet.  If you assign 172.16.0.1/255.255.0.0 to interface Gi0/0 it means that the entire 172.16.0.0 network is directly connected to interface Gi0/0.  If you then assign 172.16.0.80/255.255.0.0 to interface Se0/0/0 it means that the same entire network is directly connected to interface Se0/0/0 as well, and you can't have that because the router won't know on which interface to look for destination IPs in that network.

Using IP range .0-.63 with netmask 255.255.255.192, .64-.79 with netmask 255.255.255.248 and .80-.83 with netmask 255.255.255.252 would be the proper way to do it.

[ripnlipsco]

See i was under the impression thats what i needed to do as well but when i try lets say for the .0-.63 network i open the computer in packet tracer, it asks for ip address i input 172.16.0.63, then it asks for subnet mask when i enter that which you provided it says "invalid IP for this subnet mask entered" Thats why im stuck. It also asks for default gateway i entered the ip of the first router. Not that it matters Do you now why it gives me that error when i enter that

[Iulian Vaideanu]

For subnet 172.16.0.0/255.255.255.192 .0 is the network address and .63 is the broadcast address - that's why you cannot assign it to an interface.  Same for 172.16.0.64/255.255.255.248 (.64 = network, .79 = broadcast) - in general first IP in the range is the network address, last one is the broadcast.