Netmask/DNS issue

Posted on 2015-02-01
Last Modified: 2015-03-06
I needed to expand the IP addresses available for our internal use so I was going to add an additional sequential subnet.   The current subnet mask is  I went to network connections on the server, selected the local area connection and changed  the netmask to and now I can not access the server by name from some computers.    

When I changed the netmask from to it removed it from some of the DNS servers.  It finally updated after 45-60 minutes.  

I changed it on another 2008 server without any of these issues.

should changing the netmask on a 2008 64bit server cause this issue?
Question by:qvfps
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LVL 79

Expert Comment

ID: 40583155
Net ask changes must be propagated to all systems.

The likely issue is that the ip a system got based on the 252 sees the server address as local, while the server ip/mask combination does not.

Once you dive down this route, you have to update the dhcp server scope definition to push out the new mask.

A better approach is to define another ip range on the router with an ip helper dhcp relay agent configured to send requests to the dhcp server where you would define another scope iof IPs.  The router will then route between the segments.
You went from 2x256-3  possible hosts to 4x256-3possible hosts

Author Comment

ID: 40583177
My intention was to change the NetMask on the comptuers/servers with fixed IP addresses then update the DHCP scope.
LVL 79

Accepted Solution

arnold earned 250 total points
ID: 40583188
The issue is that changing the scope is not that simple, your best bet would be to export the current config using netsh dhcp server dump/export

Alter the netmask/scope and import it back.

This way you will maintain any IP reservation you may have as well as IP range exclusion

If you are increasing from a 509 possible usable IPs/host to a 1021 there are many devices where this change Needs to be done including VPN/firewalls if any, managed switches, etc.
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LVL 70

Expert Comment

ID: 40583451
To make sure we are on the same page: The netmask is now?

Author Comment

ID: 40585440
Currently the netmask is but I am running out of available IP address to assign via DHCP.  
DHCP range
Fixed IP range

I requested more IP address from our corporate office and was told I could use which had been assigned to us before I started at the company but never used.  

I had planed on updating the netmask on the servers/printers/switches to then updating the DHCP to use as the netmask for assigned IP addresses and finally if I ran into no problems testing fixed ip addresses in the range adding a new pool to the DHCP server.

I changed two servers last night and one disappeared from some of the DNS servers for close to an hour.   I changed both back to after that.  

We already have multiple VLANS, multiple stacks of switches connected via fiber, a separate outgoing internet connection for guests and both wired and wireless routers and a definite lack of labeling and documentation which I am slowly trying to remedy.    I was trying to avoid adding any more complexity to the network
LVL 79

Expert Comment

ID: 40585463
You are citing a netmask that is not valid. means there are only two IPs in the block, 252 means there are four. These types of netmasks are only used in firewall rules (254 to allow two sequential IPs through the firewall or deny) the 254 or /30 notation is used on a wan connection.
You can have multiple scopes defined in the Dhcp, or super scopes, but you would need to separate the networks/ports by assigning them to separate VLANs and then configuring an IP helper/DHCP Relay agent such that when it proxies the DHCPdiscovery request it will reflect the request such that the response of IP from the DHCP server would be from the range.

The router or the switch would then need to be configured to handle the inter segment communication/routing.

Author Comment

ID: 40594387
sorry you are right about the netmasks.  They should have been 2552555.252.0 and
LVL 70

Assisted Solution

Qlemo earned 250 total points
ID: 40599008
Changing the netmask can have almost any effect with your configuration, but that is something only you can know or get to know :D. In theory, and with a well configured and documented network, all potential point of failures would be easy to spot. Since you have VLAN, you need to set routing info in the L3 switches correct. Aside from that, nothing obvious should prevent you from changing the netmask.

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