Subnetting on a network

sendalot

So, on subnetting a network, when shall two hosts be in different subnet?

If three routers are connected in a single office, do they all need to be in different subnet?

Does each port from the router that creates a broadcast domain, need to be in different subnet?

Then in the case of WAN(i.e VPN), where 1 router in LA, 1 router in Chicago and 1 router in New York, do the routers need to be in the same subnet?

I'm trying to discern when it's ok for hosts to be in the same subnet. (Of course hosts connected to a switch all need to belong to the same one).

Thanks.

Node Man

Routers connect a LAN to the WAN. I would imagine that a single (small) office would have a single router and then maybe 3 switches in various physical locations or all in the same closet. I think your question may be answered by using vlan trunking.

iamme4eva

Each interface or sub-interface on a router should be in different subnet. For a multipoint WAN topology, you would have a subnet allocated for your WAN links. Each LAN interface connected to the routers would be a separate subnet. If you started using the same subnet off two different interfaces of the same router, you wouldn't be able to route between them. Even if the router allowed you to configure two ports in the same subnet (most IOS versions don't), then you would have two directly connected routes to the same subnet in your routing table, which wouldn't be helpful! Each VLAN should also have it's own subnet, with a router routing between them, if required.

Also think about this: when a host A tries to ping host B, it first checks to see if the destination address is in it's own subnet. If it is, then Host A will believe that it should be able to communicate directly with Host B. It will ARP to find the MAC address of host B. The router (unless configured to transmit broadcasts...another subject), will not pass on that ARP request, so the ping will fail. Host A will never try to contact it's default gateway when trying to contact another host that it believes to be in it's subnet.

I hope that makes sense - I'm tired!!!

sendalot

So, in terms of WAN, routers in two different geographical location may or may not be in the same subnet?
Thanks.

Node Man

The routers would be in their own little subnet with eachother, and they would each have a different subnet facing their lans. There would be at least 3 subnets.

pert

Example:

Router A:
Internal Supernet for Site 1: 10.1.0.0 /16
WAN Link to RtrB : 10.100.1.0/30

Router B:
Internal Supernet for Site 2: 10.2.0.0/16
WAN Link to RtrA: 10.100.1.0/30

(A Network) <- Rtr A -> Wan Link <- Rtr B -> (B Network)