vlan doubt

If they don't match the IP of the router interface in the VLAN, then they won't be able to talk off their subnet. And since you don't want huge broadcast domains and you do want logical network design, you don't want to have a gigantic network range. Therefore, they must be in the same range as the router interface.....unless of course this is a completely non-routed vlan..of which case it shouldn't matter so long as its all LAN.

because when a packet has to go between two different networks it has to be routed.. as soon as you put a router in theer to do that they are no longer on the same subnet. when all devices are on the same network address devices can broadcast packets to everyone and arp correctly.

Flamer.

think of a vlan as a network. why do hosts in the same network have the same network id?

This isn't a "must do" thing. A VLAN is like an isolated ethernet segment. Think of it as a coax cable run in the early days of ethernet.

You can have multiple subnets co-exist in the same ethernet segment. Nodes in one subnet can't talk directly to nodes on another subnet even if their ethernet packets can reach each other. And all nodes will see all the broadcast traffic, but nodes in one subnet will discard packets that are not meant for their subnet. (But some nodes might be multi-homed with an IP interface for each subnet and hence be able to talk to nodes in either subnet).

In the end, you have one ethernet segment with traffic from 2 subnets, broadcasts from 2 subnets etc. By segmenting this into 2 VLANs, you reduce the traffic on each VLAN, without impacting the nodes in each subnet.

If one node/router has interfaces in both subnets, you can define its port as being member of both VLANS. That node sees the traffic from the

One possible reason to have multiple subnets on the same VLAN segment is when you have multiprotocol nodes. (TCPIP, SCS, LAT, DECNET, APPLETALK etc)

Say your have a VMS cluster that uses SCS (ethernet) to communicate between nodes. Those nodes need to be in the same VLAN. But 2 nodes might have an IP stack with an "internet" address that respond to TCPIP traffic from the internet, and 2 other nodes might have IP interfaces that are in a local non-routable IP range such as 10.*.*.* to serve local corporate requests.

VLANs are a tool. It creates virtual ethernet segments that are isolated from each other. There are many advantages to this. But it doesn't mean that it must be used in any specific way. You need to understand what protocols you are using on your LAN and decide whether it is best to separate traffic or just keep it all on the same logical ethernet.