Subnet/DHCP Scope Size

[dave.81]

We've been running on 255.255.252.0 for a few years now but as the amount of devices has grown and grown. We have today maxed out the subnet and need to expand.

 

If we moved to 255.255.248.0 this would give us a lot more but what is needed in the process? I know i'll need to change all the static devices over to the new subnet, do i then just add a new scope for the subnet and remove the old one?

[foofighterjim]

I know i'll need to change all the static devices over to the new subnet, do i then just add a new scope for the subnet and remove the old one?

 

In theory yes, but in practice that leaves you with a very large broadcast domain and you will eventually see your network swamped with broadcast traffic (Especially if you use Apple devices or have Bonjour enabled on your printers). It might be a good idea to start looking into VLANs.

 

Edit: I was in a similar situation last summer; I managed to sort it out with the help of several kind folk here: http://www.edugeek.net/forums/how-do-you-do/156740-subnet-change-vlans.html

Edited January 29, 2016 by foofighterjim

[Michael]

In terms of DHCP Server, you need to delete the Scope and re-create it from scratch with the new subnet.

[twin--turbo]

I am with FFJ..

 

I run /24's and vlans keeps broadcasts contained.

 

TT

[jezi55]

I am also in the same boat, currently dhcp has about 70 free IPS when everything is on. I was looking at the .248 subnet but I was under the impression every single switch and server will need its subnet changing. I have about 50 switches and servers throughout the school. Out of curiosity what would happen if I missed one of the switches? Would everything on there just work as normal as all my current switches and servers are in the old scope?

[twin--turbo]

I am also in the same boat, currently dhcp has about 70 free IPS when everything is on. I was looking at the .248 subnet but I was under the impression every single switch and server will need its subnet changing. I have about 50 switches and servers throughout the school. Out of curiosity what would happen if I missed one of the switches? Would everything on there just work as normal as all my current switches and servers are in the old scope?

 

unless your doing any routing with vlans, which by the sounds of it your not. then the switches don't realy matter as the IP/subnet is just for management.

 

 

With a network of 50 switches you need to be vlanning to cut that broadcast and to break up traffic.

 

TT

[Muz]

I'd go no bigger than a /23

 

You need VLANs for sure

[Michael]

I am also in the same boat, currently dhcp has about 70 free IPS when everything is on. I was looking at the .248 subnet but I was under the impression every single switch and server will need its subnet changing. I have about 50 switches and servers throughout the school. Out of curiosity what would happen if I missed one of the switches? Would everything on there just work as normal as all my current switches and servers are in the old scope?

 

It really depends on your requirements - If you're currently using a .252 subnet and considering a .248, why not consider a .240? This'll give you over 4000 IPs compared with just 1000 IPs and will definitely future proof yourself and is suitable for a single/large network.

 

You can have multiple VLANs within the same subnet, but it's not recommended. You're just as well to have everything on the same single VLAN.

 

I always this IP Calculator to view all possibilities when configuring new networks.

[jezi55]

Cheers guys the feedback is greatly appreciated. At the moment the only traffic that is vlanned is our VoIP and CCTV traffic. EVERYTHING else just gets passed through without any vlans. Would you recommend vlanning off say the wireless and printing? (Sorry for hijacking the thread)

[twin--turbo]

You can have multiple VLANs within the same subnet, but it's not recommended.

Could you expand as I don't think that's possible on IOS, Procurve or Comware without Bodgery.

 

The only reason I would not use a /24 would be to have fun using ip addresses 0 and 255.

 

TT

[twin--turbo]

Once you start to segregate traffic it's up to you how you do it.

 

Multi Client VLANS

Server

Wireless

Printer

CCTV

VOIP

BMS

DAG/Mail replication

DMZ

External

[FN-GM]

I wouldn't go for a .248. You need to vlan off. Split everything off into subsets no bigger than /24. The amount of broadcast traffic must be immense!

[dave.81]

Thanks for the info. VLANS is something i want to be moving towards but the school has been cabled in such a haphazard way will need investment in switchgear to put into place, i think.

 

Maybe it's time for that difficult conversation about the infrastructure and how it needs a major overhaul!

 

Edit: I'm a complete newbie to VLANS, if i moved the wirelss off to a vlan i need to tag each port with an access point on it to VLAN:50 say? Then set up a dhcp on the server for that vlan?

Edited February 1, 2016 by dave.81

[IrritableTech]

Just a quick note which I don't think anyone has mentioned. If you change your subnet you probably need to speak with your ISP unless you also run your edge router and know exactly how it's set up.

[dave.81]

We run our own router so can change what we need when i can put a plan in place to implement the vlans.