subnet mask / DNS

[Fred Martin_1]

I have a class C network and I'm running out of IP adresses. As a temporary measure, I'd like to broaden the subnet mask to gain some IP addreses - make it a class B network.

As I understand it, I should be able to modify the subnet masks of hosts 'on the fly' without any downtime - i.e. if I change the SM to 255.255.0.0 on any host, but leave the IP address unchanged, the host can still network with all the other hosts that have the class C subnet mask.

However, DNS seems to fail. My DNS server is an HP-UX box running Bind. Am I missing something, maybe something to do with broadcasts?

[Geoff Wild]

Wow - class b - why not just add 1 more c?

255.255.254.0

Anyways - it may be the default gateway - make sure your DNS server can see it - and you may need to restart bind.


Rgds...Geoff

[Fred Martin_1]

True I don't need the whole octet :)

I hadn't restarted DNS because the DNS server was unchange.

The HP-UX box running Bind still has it's original IP address and SM 255.255.255.0, all I did was change the SM of one host to 255.255.0.0, kept the same IP address on that host. So the final setup is:

DNS server
IP: 192.168.100.15
SM: 255.255.255.0

Host
IP: 192.168.100.122
SM: 255.255.0.0

The plan was to change all the SM's on all the hosts, without changing any IPs.

From the IP standpoint it's fine, my test case worked - the two boxes can network OK.

But the host can't resolve names any more (and because of that, can't authenticate with Windows, etc).

[Fred Martin_1]

Well probably I'm wrong. DNS does appear to be OK, as I can open a command shell on the PC host and ping another host by name.

I just can't authenticate with the Windows NT domain.

Maybe, if the subnet mask is changed on the Windows NT PDC first, the host would be happier.

[Fred Martin_1]

Ok, it does have something to do with NT.

I changed the subnet mask on my PDCs and BDCs, and now the PC hosts can authenticate.

I really don't know why, except that maybe if the NT servers are SM 255.255.255.0, and the hosts are SM 255.255.0.0, then broadcasts needed for NT Authentication are being thwarted.

Again, eventually they'll all have the same class B subnet mask, anyway.

[Geoff Wild]

Fred - yes - Windows is rather picky when it comes to networking - glad it all worked out for you. :)



Rgds...Geoff

[Fred Martin_1]

Well, to complicate it further, my DNS server is HP-UX, and yesterday I changed the SM on that machine, to 255.255.0.0.

Following that, none of the XP hosts still having the 255.255.255.0 mask could resolve names, even though they could network fine.

Change the SM to match the DNS server and it works.

(remember, no IPs have changed yet)

I really was hoping to change the SM's during up-time since I was widening the network and not making it more narrow. Looks like that's not going to work.

I'm quite sure there was a day (pre-Windows NT/XP) when that would have been fine.

[Geoff Wild]

You know what you could do - plumb up a second ip address in the "new" network on your DNS server - that way it can serve both subnets.

Rgds...Geoff

[Fred Martin_1]

Baaah. This has to be a WinXP thing.

XP host:
IP 192.168.100.123
SM 255.255.255.0

HP-UX DNS server:
IP 192.168.100.66
SM 255.255.0.0

If I open a command shell on an XP host, and type 'ping dnsserver' it fails to resolve the name dnsserver.

But I can ping dnsserver by IP address, and browse the web via our gateway which is SM 255.255.255.0 - those names put into IE resolve fine.

Anyway - easiest thing - I'll just spend Sunday down-time cutting over -all- the hosts to the new SM, and all will be well.

Thanks for your input Geoff...

[Fred Martin_1]

The 'Baaah' wasn't directed at you :) it was for Microsoft...

[Dino_4]

Hi,

back to networking.

You use now the ip-address range 192.168.x.x.
This is a C-class network, as you said.

You can't just change the subnetmask to a B-Class without changing the ip-adresses of all hosts to a B-class address.

The solve your problem just change the subnetmask to 255.255.254.0.
With this you have 508 hosts to address, since you said, you're running out of addresses in your 255.255.255.0-net, you now have around 255 hosts to adress. So that would double your net and should be sufficent.

The problem i think, is the broadcast address.
It is diffenrent in your scenario for the DNS and the host.
For the DNS it's 192.168.100.255.
For the host it's somewhere at 192.169.155.255

[Dino_4]

sorry, i mean 508 hosts to address with SM of 255.255.254.0

[Fred Martin_1]

Well, here's an example:

IP 209.90.236.66 <-- HP-UX DNS server
SM 255.255.0.0 (class B)

IP 209.90.236.77
SM 255.255.255.0 (class C)

I have machines on the same physical network addresses as you see above. They can in fact communicate, i.e. they can ping each other, I can telnet between them.

Under those conditions though: when .77 is a Win98 host, it can resolve DNS names. When .77 is a WinXP host, it can not.

That's what I'm experiencing at the moment.

Again, I do not intend to leave hosts set up in this way. Eventually all the subnet masks will be the same. I'm trying to avoid have to change them all at once during downtime.