Expanding IP range

Confused · 25 Sep 2007 at 12:28

Hi folks,

Multi-site network at work is starting to get full, and we're running out of IP addresses on some subnets! The bugbears of when the network was set up ~7 years ago, when there were only a handful of computers, and people didn't think that we'd have CT/MRI/Xray scanners attached to the network and sending diagnostic imaging all over the hospital!

Basically, we have:

IP: 10.1.1.x - Mask: 255.255.255.0 - Site A
IP: 10.1.2.x - Mask: 255.255.255.0 - Site B
IP: 10.1.3.x - Mask: 255.255.255.0 - Site C
IP: 10.1.4.x - Mask: 255.255.255.0 - Site D

Each site is physically it's own seperate network, but with routers connecting them via leased line. Site A (10.1.1.x) is the "centre" of our network - each other site is connecting into Site A.

We're rapidly running out of IP addresses, and we have lots of equipment on static IPs at each site - of course the simplest would be to take everything to 10.1.x.x and 10.2.x.x etc with mask of 255.255.0.0 (to take us from 254 hosts to 65534, but changing these IP addresses/subnet masks isn't going to be an easy (or cheap!) task.

After having a quick look, we've come across what Microsoft calls a "superscope" for DHCP, if I understand correctly, it allows us to have multiple DHCP scopes of different subnets attached to the same physical network, allowing us to add additional subnets to the current ones, without having to re-address all our existing equipment. However, I'm not too sure whether this is right, and additionally, how all the equipment on one site would talk to each other.

Any help/advise would be greatly appreciated, it's been over 5 years since I did any network design/subnet calculations, and my head's gone into meltdown trying to regurgitate what I had previously learnt!!

Cheers!

Garry

kirkster · 25 Sep 2007 at 12:50

Hi there,
Yes, that is quite possible. You can introduce other /24 networks onto your core site so that, for example on site A, as well as the 10.1.1.0/24, you could add a 10.1.5.0/24 (your next freely available subnet). This would avoid readdressing requirements (which is always a nightmare). However, you'd need to intoduce either a secondary interface on your router, or a vlan subinterface or else you wont be able to connect between the two subnets.

In Cisco parlance this would be:

interface f0/0
ip address 10.1.1.1 255.255.255.0 ; your primary interface
ip address 10.1.5.1 255.255.255.0 ; your secondary

OR (using subinterfaces and vlans)

interface f0/0
ip address 10.1.1.1 255.255.255.0 ; ypoutr primary (native vlan)
interface f0/0.10
encapsulation dot1q 10
ip address 10.1.54.1 255.255.255.0
no shutdown

You'd then either need to add the secondary to your router statements or define static routes on the edge sites.

Hope this helps

Steve
CCIE #11330

Confused · 25 Sep 2007 at 12:59

Thanks for the prompt reply.

We would, at the first stage, move all the DHCP'd client workstations onto the new subnet (as per your example 10.1.5.0/24) , but all the servers would remain on the old subnet (10.1.1.0/24) - would this mean that all traffic between the two /24 subnets on a single site would need to go via the router's interface?

Garry

kirkster · 25 Sep 2007 at 13:09

Hi there,
Indeed, all traffic would need to flow throught the router. This would mean that a PC in the 10.1.5.0 subnet, talking to a server in 10.1.1.0 needs to go though the router, either the "router on a stick", in and out basis as I described before or by using a another router interface, connected back to your switch stack connecting to the router.

This is perfectly feasible in design terms. However, if you have large traffic volumes flowing between these subnets (and radiology images are often many tens of megabytes in size - as I know since I do network design as my job) you may need to look at deploying a L3 capable switch such as a Cisco 3560 or 3750 that is capabale fo routing millions of packets per second.

What is your campus switching infrastructure and what routers do you use? If they are Cisco what is the IOS release?

Steve

Confused · 25 Sep 2007 at 13:16

Switches are currently HP ProCurve, mixture of 4104GL frames, 2450, 2650. Routers are Cisco, show version gives: IOS (tm) C2600 Software (C2600-DS-M), Version 12.1(3a)T1, RELEASE SOFTWARE (fc)

kirkster · 25 Sep 2007 at 13:24

Hi there,
Those routers are as old as the hills and the software is unsupported. The performance of the bog standard 2600 will lead to disappoiting performance if you interconnect the subnets on a router on a stick basis. However, you can still run a secondary interface on the ethernet port. The address you enter will be *** default gateway of *** net subnet. As I say this will work but performance will not be great between *** two segments since software based routing is only in the 000's of packets per second. That sounds a lot but belive me it isn't, not in busy hospitals with mega images being sent here there and everywhere.

Do you have any money available to buy equipment? What leased lines do you use? X21 or LES 10/100 ? You could introduce a L3 switch to act as the default gateways and do a bit of readdressing on the WAN links. Is down time easy to get?

Steve
Got to go back to work now but will check later....

Confused · 25 Sep 2007 at 13:47

Steve,

Thanks very much for your help/advice. I will pass the link to this thread on to my boss, and get his input on the situation now I have the knowledge you have provided!!

Our leased lines are X21, but we have had surveys done for RF links between the sites, replacing the BT leased lines - so the routers would likely be changed anyway for ones with ethernet links, rather than serial, to facilitate the new WAN equipment.

Budget, unsure - boss deals with money side of things!! Down-time would likely be a late-evening/weekend thing only, but we could do it.

Garry