CCNA Subnet question -- Different networks on the same segment

caomhino

Hi there, currently studying for the CCNA and i have this query.

If i have serial link between two routers (or even ethernet), why do both ends of the link have to be in the same network.

Now i know routing will never work, and i'm not exactly sure why ???

I've set it up on packettracer, L2 comes up fine, up/up, at each end but i can't ping across. I have static routes set ok on each side but ping doesn't work. Is it something to do with the ARP process not completing ? cheers.

i.e R1 interface is 192.168.1.1/30
R2 interface is set to 192.168.2.1/30

any ideas.

ntlbell

How does one router get to another router that's on a different network?

via another router on the same network...

So yea, they need to be on the same subnet/segment

_CreeD_

ARP is an ethernet protocol, so forget it on Serial (and point to points usually don't rely on L2 addressing anyway, though some allow you to specify or have similar L2 to L3 protocols depending on implementation).
Subnetting is effectively how you teach your device whether a target host is on the same L2 network or not. by placing them in different subnets you have told them both not to use L2, and since they have no L3 reachability you're dead in the water.

Razzen

Ok i'll try explain whats happening. without getting into Layer2, layer 3 stuff.

When you want to send a packet from A to B. You compare your own network address and the destination network address, you want to see if the destination address is on the same network as you. If it is we can just send the message and assume that if its on the same network it will be picked up.

But what if we compare the source address and destination address and realise that they are on different networks? what do I do now? In this case I have to send the packet to my default gateway address and hope that they can find a way to get to the destination network.

If we use the values you suggested and assume I was connected to R1 and was trying to get too 192.168.2.1/30 on R2. So I'm assuming that host A is on my LAN and it has sent the packet to its default gateway on its local router R1 trying to find a route to the destination network. The router now checks its routing table to see if it has any routes that match the destination address. This is your problem, The routing table will only show 192.168.1.1 and not the 192.168.2.1 network, so when my router R1 tries to forward the packet to B it can't find a route for it in its routing table and hence won't know where to send the message and will drop it. so it never even sends it out the serial interface.

there is a bit more going on but i hope you can follow that and it helps answer your question.



i.e R1 interface is 192.168.1.1/30
R2 interface is set to 192.168.2.1/30

axer

OP: Do a show interface for the serial interface and show route on both routers and post here or alternatively post both configs here and we can see why.

_CreeD_

Em...it's been explained why 3 times now in slightly different ways . Those details aren't going to change anything.