extending a subnet

CaptainGiblets used Ask the Experts™
I asked a question about extending my 192.168.20.x subnet to encompass 192.168.21.x as well and was told to change the subnet mask to

I am just about to do this and just wanted to double check, what should be the subnet mask on the 192.168.21.x machines? Would it also be 255.255.2544.0 or  I would presume the first one, but then how would those clients know to look for .20 and not .22?
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Address:         11000000.10101000.0001010 0.00000000
Netmask: = 23   11111111.11111111.1111111 0.00000000
Wildcard:            00000000.00000000.0000000 1.11111111
Network:      11000000.10101000.0001010 0.00000000
HostMin:         11000000.10101000.0001010 0.00000001
HostMax:       11000000.10101000.0001010 1.11111110
Broadcast:       11000000.10101000.0001010 1.11111111
Hosts/Net: 510                   Class C, Private Internet

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Subnet and mask should be identical on all network hosts in this subnets


Ok i have changed my subnet mask to

on my servers i have changed the subnetmask to above and all servers and pcs are in the .20.x range

I have changed my phone system (which i could previously ping) and one phone to the .21.x range.

Now my phone and the system can communicate, however i cant ping them from my .20 pcs.

What could be wrong?

On all servers / pcs i have

ip 192.168.20.x

on phone system i have
ip 192.168.21.x

all 20 and 21 hosts are in th one sife of router, or or in a different sides of old router between 20 and 21 subnets?
maut be in one side
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they are all plugged into the same switches, the only gateway on that is an isa server which is in the .20.x range with a 254.0 subnet mask. I have also extended the isa server to show .21.x subnet as internal.


i was just reading back across my old question


The switch that i have is only a layer 2, would this be the reason why they cant talk between eachother? Or would it be something different? As from what i read it says that they cant talk between 2 different subnets? However are they not classed as the same subnet? but a class B not C?


Sorry for all the posts, but trying to keep you up to date on what i have / havent done.

Read more into the switch situation and it shouldnt be a problem.

Also, i have run a tracert from one of my pcs with the subnet on the 192.168.20.x range. The connection goes to my isa server (default gateway) and then ends there.

The isa server is configured with the new subnet mask and has an ip of

Now i understand it is the default behaviour to send packets to the default gateway if its not part of the same subnet, however as they are why is this occuring?
It sounds like you have different VLANs created on the L2 switch or something still has the mask wrong. With the wider mask all the devices need to be on the same VLAN as well as in the same network or they will try to hit the gateway to get to the other network.

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