Question about changing a Snow Leopard Server network settings

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Level 1

20 points

Question about changing a Snow Leopard Server network settings

We have an Xserve running Mac OS X Server 10.6.8, the primary services running on it are afp, smb, dhcp, dns, jabber and the calendar sever.

Right now we have a router connecting two networks together, 192.168.0.0/24 and 192.168.1.0/24. The Xserve has a static IP address in the 192.168.0.0/24 range.

In order to combine two phone systems into one I am required to combine the above networks, this will involve removing the router and changing the subnet mask on every device from 255.255.255.0 to 255.255.254.0, any device that is currently part of the 192.168.1.0/24 network will also need to have it's router setting changed from 192.168.1.1 to 192.168.0.1. That part should all be easy.

However, this is my first experience expanding a network past a standard class C and am wondering if there is anything I need to watch for on my Snow Leopard Sever.

Specifically I wonder about dhcp settings, right now there are two dhcp servers, the Xserve which provides dhcp for 192.168.0.0/24 and the router that provides dhcp for 192.168.1.0/24. Once the router is removed the Xserve will need to provide dhcp for the entire network.

Looking at the Xserve dhcp configuration right now I have one subnet defined providing addresses 192.168.0.5 to 192.168.0.125.

Can I simply click the + to add another subnet and add for instance a range of 192.168.1.5 to 192.168.1.125?

Will it simply give out all of the ip addresses in the first range and then start with the 2nd? Or do I need to find a single range of ip addresses large enough to meet my needs? (e.g. 192.168.1.5 to 192.168.1.225 or something).

Is there anythign else to watch for after making this type of network setting change?

Thanks in advance.

Posted on Aug 14, 2012 8:32 AM

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Answer 1

Aug 14, 2012 9:50 AM in response to roarkh

FWIW, the classic old "Class C" stuff has been dead for some years now, it's all CIDR now. The 255.255.255.0 subnet mask is now a CIDR /24 subnet network, and 255.255.254.0 is a /23.

If you're going to roto-till your network, then do yourself a huge favor and get out of 192.168.0.0/24 and 192.168.1.0/24, as those two subnets are used with many home networks and coffee shops and such, and having the same subnet on both ends of a VPN connection messes up IP routing; VPNs won't work.

If you move to 172.16.0.0/12 (the old private Class B) or 10.0.0.0/8 (the private Class A), then you can set up your own subnets, however you want, and you can avoid colliding with other folks that use the 192.168.0.0/24 and 192.168.1.0/24 subnets if (when?) you get to using VPN connections.

Yes, you'll need to tweak your DHCP server(s) to pass out your new gateway address, your existing or new DNS server address(es), and your new subnet mask.

DHCP clients don't know what subnet they're going to be getting, so the DHCP server(s) has to pass those out based on the network controller that the DHCP request is arriving via, or via the hardware MAC address provided by the client. If you're uniting the network into one (bigger) subnet with one DHCP server, then you'll likely end up with one (probably bigger) DHCP pool.

And while you're at it, it's fairly common to maintain subnets here, as making one (bigger) network can lead to far more traffic can arise at some parts of your network. Subnetting partitions that traffic. This configuration partitioning can be achieved with managed switches and virtual LANs (VLANs), or by physical wiring and IP routers.

One possible downside of a /23 configuration: Not all IP network devices can deal with it. Some low-end and some poorly-implemented IP devices expect and are locked to a /24 designation.

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Answer 2

roarkh Author

Level 1

20 points

Aug 14, 2012 12:42 PM in response to MrHoffman

Thank you for your very detailed reply.

I understand the problems that can arise using 192.168.0.x and 192.168.1.x and if this were a new project I would definitely switch to something else, however in this case we have been using these ip ranges for years (they were in use here long before I started) and I don't want to have to redo all of my dns settings at the same time I make these other changes so I'm going to stick with the same ip's we are using at least for now. Anyone here needing vpn access has already dealt with this issue but we have very few vpn users so it has not been a major problem for us to date.

My biggest question is about increasing the size of the dhcp pool. For instance, after I change all the network settings and set up a /23 network is it possible to have a single dhcp pool that includes some ip's in the 192.168.0.x range and others in the 192.168.1.x range? I guess what I am asking is when I am in Server Admin and click on DHCP one of the tabs across the top is "Subnets". Right now there is a single "subnet" configured that provides addresses from 192.168.0.5 to 192.168.0.125.

Ideally I would like one dhcp pool that provides ip's in the range of 192.168.0.5 to 192.168.0.125 and then if all of those are used up, moves to 192.168.1.5 to 192.168.1.125. Can I do that by simply adding another "Subnet" so there are two? Or do I have to find one larger single range? (e.g. 192.168.1.5 to 192.168.1.225?) You may have already answered this indicating I have to have a single larger range but I just want to be sure.

The parts about changing subnet masks, and routers and all that I think I have a pretty good understanding of.

Your last paragraph is something I have been worried about, I do have a feeling that after making this change there will probably be unforseen issues that will need to be resolved, I'm just hoping it is not too many. For the scope of this thread I will just focus on issues that may arise with the Snow Leopard Server however.

Again, thank you for your help.

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Answer 3

Aug 14, 2012 2:34 PM in response to roarkh

What you want isn't feasible with the command tools available with OS X Server; either with the GUI or with the bootpd tool at the command line.

The OS X Server DHCP server doesn't support non-contiguous pools of IP addresses. You get one contiguous pool of IP addresses per network interface controller (NIC).

You can use VLANs and segment with managed switches (and likely including bridging the VOIP stuff, or whatever kicked this all off, depending on exactly what sort of VOIP traffic is occurring here and what sort of managed switches you have access to), or can maintain the existing separate subnets, or investigate a DHCP server that can deal with multiple pools.

And FWIW, there's never a good time to fix an address space, or to simplify your configuration. Never. If DNS server settings are what is preventing you from making a mass IP change, you can shut down DNS services and edit the configuration files (carefully) directly.

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Answer 4

roarkh Author

Level 1

20 points

Aug 14, 2012 3:36 PM in response to MrHoffman

Ok, thanks, since my last posting I took a look at my network and realized I have plenty of ip's available in the 192.168.1.x range so I will just use those as my dhcp pool.

As mentioned before I do agree that it would be nice to change the ip addresses, however we have been using these ranges for years and it really is not causing us any major problems, there have been perhaps once or twice where I have had to help one of our users make a change to their home network but it is so infrequent as to really not be a problem. So I just don't see a reason to move from the ip ranges we are using now to different ones unless there is some other compelling reason.

Thank you again for all your help.

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