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I'm a programmer by profession and don't understand this stuff all that well, so I apologise if this question is unclear, and I will try to keep improving it.

I have a /24 IPv4 network, so approximately 255 IP addresses. Very soon we will have more than 250 devices on the network, and this number will continue to grow over time, therefore I have to expand the network capacity somehow. I am already making heavy use of DHCP to reduce the number of devices using an address at once.

I can increase the size of the subnet to a /22, giving ~1024 IP addresses. If I do this, will there be issues with having so many devices on a single network? Should I split it up into several smaller subnets? How would I achieve this?

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Make sure you are also planning to deploy IPv6. You won't be able to get IPv4 addresses forever, or even for very much longer. – Michael Hampton Sep 20 '12 at 11:09

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up vote 7 down vote accepted

Normally, I would advise using subnetting and multiple VLANs, but if you don't have a network administrator, and they have to press programmers into service to fill the role, it's probably wiser to expand the subnet to a /22 or /23. Much simpler, and you won't need to divert programmers to administering network configs nearly as often with the simpler approach.

Technically, Cisco still advises not using anything bigger than a /24 subnet for a single subnet, but if your gear's all reasonably modern, and you don't have any bad device misconfigurations that would be flooding the network with broadcast traffic, you won't see any issues from going up to ~500 devices on the same subnet, and might even be able to get away with ~1000. (That's as far as I'd push it, though.)

If you grow out of your /23 or /22, you'll need... or at least REALLY want to hire a network administrator, or least a consultant to set up a multi-subnet, multi-VLAN network environment for you, if not administer it as well.

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+1 for taking into account that the OP doesn't have the knowledge to set up VLANS. – John Gardeniers Sep 20 '12 at 12:35
As for Cisco's recommendations, I actually asked this as a question not that long ago. I had always heard/read that it was 500 nodes, but could never source anything in-depth/conclusive as to why. – gravyface Sep 20 '12 at 12:36
We actually do have a network administrator, but he's overworked and doesn't have time to look into this at the moment, so he's asked me to do a little research just to ensure that we're going about this the right way. We were thinking multi-subnet was the way to go, but wanted to make sure it was necessary. It sounds like it's the best path. – MikeFHay Sep 20 '12 at 12:37
@gravyface I posted a reference to that Cisco recommendation not so long ago myself. It doesn't seem to be well documented on the web, but the reason is basically that as the subnet size grows, the amount of traffic a broadcast consumes expands non-linearly. Faster modern networks can handle the increased traffic better, but they haven't bothered updating the recommendation, because you should be setting up your network into small, manageable segments anyway. But 256 devices isn't so bad for no admin/overworked admin. I did a few thousand as the everything admin. THAT was a nightmare. – HopelessN00b Sep 20 '12 at 12:42
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@MikeL Honestly, if your network admin needs you to research for him to confirm that a multi-subnet approach is the best, then no offense to him, but he's not up to the task of administering such a setup. Maybe he's not experienced or knowledgeable enough, maybe he's too busy, maybe it's a little of both, but it sounds like he's walking himself into a mine field with this. – HopelessN00b Sep 20 '12 at 12:47
up vote 3 down vote

Moving to a /22 should not cause any problems on a modern network.

If you used old fashioned 10baseT networking with one cable running past all computers then this would be different. Only one computer could be active at any time. But with modern hardware (Read: switches and not hubs) this should not cause any problems.

The easiest way to do this is a single /22.

Multiple subnets will also work and that will allow you nice things such as:

  • All servers in one net
  • One net dedicated for management (DRAC, ILO, switches etc)
  • One net (e.g. a /23) for office computers
  • etc etc

The downside for this is slightly more complexity. If that is worth it or not is up to you.

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up vote 2 down vote

I would rather implement several smaller networks using VLANs. It will give you much greater scalability and flexibility, as well as increase security. Having ~1000 hosts in a single broadcast domain is asking for trouble IMO.

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