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New Member

Network way too big

I have a network with with devices all over that range. How do go about fixing the network without going back to each computer and reassigning an address and netmask. Can I or how do I vlan a network with the same net mask? I have the following cisco 2620xm and cisco 2950 and a ton of cheap 48 port switches.

New Member

Re: Network way too big


Do any of the switches support VLANs or Trunking? If not you might have a bit of a problem. If they do not support like 802.1q vlans you will have to physically move some of the clients around or change there ip's anyway.



New Member

Re: Network way too big

The cisco 2950 does. Is there a way to vlan by port even if all the clients are on same subnet?


Re: Network way too big

You normally don't setup VLANs for hosts on the same subnet.

VLAN=broadcast domain=subnet

My suggestion:

1. Hopefully you are running DHCP on your network. Set the lease time to be real short (like an hour) and wait a day or two. This will force the clients to keep requesting DHCP service more often than the default of 3 days (in Windows NT). Hopefully your network can handle this increase in traffic also.

Setup a new DHCP server (so your current one is not changed) When you are ready to implement the new IP scheme, you will disconnect your existing DHCP server from the network and connect this new one to the network. Setup mulitple DHCP scopes on this new DHCP server but don't connect it to the network yet:

scope 1 -

scope 2 -

scope 3 -

If you aren't running a DHCP server, this would be a good time to set one up and set your clients to use DHCP.

2. Configure an unused fast ethernet port on the 2620 for trunking and DHCP relay support:

interface fa0/0.1

encap dot1q 1

ip add

ip helper-address

interface fa0/0.2

encap dot1q 2

ip add

ip helper-address

interface fa0/0.3

encap dot1q 3

ip add

ip helper-address


3. setup a VTP domain on the 2950:

vlan da

vtp domain cisco

or whatever

4. Define VLANs on the 2950:

vlan da

vlan 1

vlan 2

vlan 3

or whatever.

5. Setup a port on the 2950 to trunk to the 2620 router. Note you must use a 100Mbps interface.

interface fa0/12

switchport mode trunk

6. Verify the trunk is up between the 2620 and 2950.

show int fa0/12 trunk


(Disconnect existing DHCP server from the network and connect new DHCP server to the network)


7. start putting ports on the 2950 into vlans.

After a while you should see the clients with the new IP addresses.

If you have multiple switches, setup VTP on each switch this way:

vlan da

vtp mode client

vtp domain cisco

then setup trunks between each switch.

My $.02

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