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Posted on 2012-03-15
Last Modified: 2012-03-30

At the moment we have a below Vlan.

int vlan20
ip address
no shut

now we do not left with the much free address on this vlan. So we are planning to subnet the Vlan from /24 to /23

In order to do and achieve this what and where the changes needs to be done.

Question by:network-guru

Expert Comment

ID: 37726560
You would then change the subnet mask used while configuring vlan. I would say it looks like

int vlan20
ip address
no shut

Author Comment

ID: 37726614
Thats fine.
My question is that apart from this where i have to make the change.

for example as earlier the Vlan10 ip address and subnet is
and this is the gateway for my LAN.

So on if there is any server in that subnet will be having the Gateway IP as and the mask information will be

anyhow it will work even if i will not change its subnet from to but it will work from the local LAN only. From outside where ever the routing was done it was learning as /24 so i need to get the routing change done as well.

so that from wherever it was getting learned as /24 will learn as /23 and to make it fullproof and make sure that it will work fine. i think it is a good practice to change the sunet as well on all host where to /23

Please correct if i am wrong.

Expert Comment

ID: 37728340
Depends if you are using static routing or OSPF/RIP etc.  You just need to change the IP address/subnet of the interface the gateway address is assigned on (  If you do this and not using static routing then the new subnet will be learned and entered in the routing table.  If using static routes you need to look on your switches/routers for these route definitions.

Don't forget if your using DHCP you need to change the scope/subnet.  DNS reserve lookup zones and if using AD, Sites subnets will need updating too.
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Author Comment

ID: 37730970
Do i not need to change the subnet mask information from /24 to /23 on each host where ever the static IP was configured on the host/servers.


Expert Comment

ID: 37731491
Yes, if you are not using DHCP and only static I addresses, the subnet mask should be changed on the hosts as well
LVL 29

Accepted Solution

pwindell earned 500 total points
ID: 37739220
Add a Router
Add a new segment.

Do not roll that mask to /23.  That creates a broadcast domain of 510 which is too big to be efficient.  Degradation begins around 300.

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