Two network segment on the same switch (no vlan), possible?

Question

Why can't 2 or more network segment work on same switch (without VLAN configuration)? It seems like switch should learn MAC addresses for each interface then work but what when I try to connnect two network segment (different network id ex.192.168.1.0 and 172.16.1.0), a lot of (or all) requests are timed out. Why? Should switch igore network id because it is layer2? I know that this might be a stupid question but I kind of confuse. Thank you

Answer 1

Hi,
it should be possible, but you need a router for connectivity or maybe a small trick.
Let us assume you have a host A 192.168.1.10/24 and another host B with 172.16.1.5/24 connected to one switch (or in one VLAN, which would give the same result).
When you f.e. ping 172.16.1.5 from host A, it will first consult its internal routing table ("route print" on a MS host). As the destination address is not local it would not send any packet unless there is a default gateway, because otherwise no route to the destination is known. The same applies to host B, when you try to reach host A. So one possible solution is installing a router and setting it to be the default gateway. Example config:
host A
IP 192.168.1.10
Mask 255.255.255.0
GW 192.168.1.1
host B
IP 172.16.1.5
Mask 255.255.255.0
GW 172.16.1.1
interface FastEthernet0
ip address 192.168.1.1 255.255.255.0
ip address 172.16.1.1 255.255.255.0 secondary
The router will get the IP packet from host A and forward it to host B and vice versa, which results in connectivity.
Another possibility is to modify the routing tables of host A and B.
host A
IP 192.168.1.10
Mask 255.255.255.0
GW 192.168.1.10
host A
IP 172.16.1.5
Mask 255.255.255.0
GW 172.16.1.5
The small trick here is that both hosts have their own IP as default gateway. This will result in host A sending an ARP for host Bs MAC, when you execute f.e. ping 172.16.1.5
As long as those ARPs are successful - and they finally should, because the switch would deliver them being OSI layer broadcasts to all ports - connectivity should be given.
Regarding your specific network problem with timed out connection attempts, I do not know your specific configuration (hosts, router), so it is hard to tell, what is going on. If you can reveal your settings it should be possible to find a solution.
Hope this helps! Please rate all posts.
Regards, Martin