Question about increasing subnet size on Vlan.

[Brandon Baker]

                   If I increase the subnet size on a vlan say for example:

vlan10 ip address 10.10.10.22 255.255.255.128

to

vlan10 ip address 10.10.10.22 255.255.255.0

Will I have to change the subnets of the devices that are currently in that vlan (router with iwithinp 10.10.10.2 255.255.255.128)?

Will I be able to add devices with ip address 10.10.10.145 255.255.255.252?

[Rolf Fischer]

Hi Brandon,

Will I have to change the subnets of the devices that are currently in that vlan (router with iwithinp 10.10.10.2 255.255.255.128)?

yes, you should do that.

Devices within the subnet with addresses < 127 in the 4th byte will still work with a /24 mask but you'll have problems with the others.

10.10.10.129/24, for example, will still receive a valid answer when it sends an ARP in order to resolve it's default gateway.

But the return-traffic cannot be forwarded by the router as .129 doesn't belong to the router's subnet 10.10.10.0/25.

Will I be able to add devices with ip address 10.10.10.145 255.255.255.252?

This network overlaps with 10.10.10.0/24, so you shouldn't use it.

On the same router you cannot assign interfaces in both networks; if it is used as a point-to-point connection somewhere else and you avoid to use this addresses in the /24 network, it could work but I wouldn't recommend to do so.

HTH

Rolf

[devils_advocate]

If you extend the size of the subnet then you will need to change the mask all the static ip addresses for hosts in that vlan, including all routers, switches, access points etc. I.e anything that has a static address in the current 10.10.10.0 /25 subnet will need to have its mask updated.

Bear in mind you can only extend the scope if you are not using the 10.10.10.128 /25 subnet otherwise they will overlap, unless you merge them.

[Richard Burts]

If I understand the original post correctly Brandon is asking if he changes the vlan interface of the switch from a /25 to /24 (increasing the capacity of the attached subnet) will he need to change the mask for the address assignment of devices already connected to the vlan/subnet. One of those devices appears to be a router at 10.10.10.2. I agree that the router mask needs to be changed since it would not be able to forward traffic to the new part of the subnet. I would suggest that best practice would be to change the mask of the connected hosts, though this may not be absolutely necessary. These devices would continue to operate ok with their gateway devices and with destinations outside of the network. The complication would come if one of the devices with the original /25 mask wants to communicate with one of the devices in the new section of the subnet. The device would assume that it was communicating with a "remote" device and would send the packet to its default gateway and not directly to the destination as it should.

I am puzzled at the part of the question that asks about using mask 255.255.255.252. Clearly that mask raises questions about what would be the default gateway for the device and I think that this would not work.

HTH

Rick