vlan subnet 2

[charliemao]

hi all,
I have a question about network vlan and vlan subnet. is it possible that many vlan share one subnet,also one default-gate?
for example, a subnet 192.168.100.0/255.255.255.0, default-gateway is 192.168.100.254. the subnet includes 3 vlans, vlan10,vlan20,vlan30 and clients in vlans get ip from dhcp server (vlan10)and if clients of different vlans get same gateway, they can be routed outside?

 

[burtsbees]

Well, the only way that can happen is via VLSM---Variable Length Subnet Mask. The entire subnet can be 192.168.1.0/24
VLAN 1 can be 192.168.1.0/28, VLAN 2 can be 192.168.1.16/28, and vlan 3 can be 192.168.1.32/28. The mask will be 255.255.255.240 for each VLAN, and 192.168.1.0 255.255.255.0 would be the "supernet" for the three vlsm segments. All are separated by logical layer three divisions, but are all part of the same main subnet. VLSM is like subnetting a subnet (sometimes referred to it as such). The router must support subnet zero (in this case) and vlsm, as well as the routing protocol. The 2 routing protocols that do NOT support vlsm are RIP version 1 and IGRP, as they do not pass mask info in the updates. Auto-summary should be turned off at the routers also. You have no need to worry about this, of course, if you simply use static routes. VLSM, however, is generally implemented in large networks, which usually means routing protocol.

HTH

Burt

 

[charliemao]

thanks burt. I am clear