Pros and cons of larger network subnets

local-area-networknetworkingsubnetubiquitiunifi

We've standard /24 subnet for users, computers and other devices. We're getting to the point where we have more users and want to go with multiple subnets for different type of devices. While this is simple to implement one of the other ways to have more devices on the network was to simply expand the network to /23 or /22 mask.

I've heard from multiple sources that it's not encouraged to use larger subnets because of broadcasting and other issues/problems that will arise from this. I've tried finding a proper source for those issues and problems and couldn't find one article (maybe I'm that bad at google) that would give me a clear pro's and con's for larger subnets, potential issues. While I understand that larger subnets such as /16 or so would be really bad for some things I am trying to understand what would be so problematic on those a bit larger than standard networks (/23 or /22). I'm also aware that having VLANs will give additional security boosts but while I can understand the pro's of having VLAN's I can't find what's so bad about larger subnets and what impact it can have on the network/devices.

Current: 

Address:   192.168.0.1           11000000.10101000.00000000 .00000001
Netmask:   255.255.255.0 = 24    11111111.11111111.11111111 .00000000
Wildcard:  0.0.0.255             00000000.00000000.00000000 .11111111
=>
Network:   192.168.0.0/24        11000000.10101000.00000000 .00000000 (Class C)
Broadcast: 192.168.0.255         11000000.10101000.00000000 .11111111
HostMin:   192.168.0.1           11000000.10101000.00000000 .00000001
HostMax:   192.168.0.254         11000000.10101000.00000000 .11111110
Hosts/Net: 254                   (Private Internet)

Planned:

Address:   192.168.0.1           11000000.10101000.000000 00.00000001
Netmask:   255.255.252.0 = 22    11111111.11111111.111111 00.00000000
Wildcard:  0.0.3.255             00000000.00000000.000000 11.11111111
=>
Network:   192.168.0.0/22        11000000.10101000.000000 00.00000000 (Class C)
Broadcast: 192.168.3.255         11000000.10101000.000000 11.11111111
HostMin:   192.168.0.1           11000000.10101000.000000 00.00000001
HostMax:   192.168.3.254         11000000.10101000.000000 11.11111110
Hosts/Net: 1022                  (Private Internet)

Would really appreciate if someone took a time and actually gave a nice overview for this.

Best Answer

There are no specific problems with larger networks, they might work properly. The issues that potentially might arise are not connected with the IP address ranges (L3), but from the number of hosts that are online within single broadcast domain (i.e. L2).

  1. There is almost no difference between having 192.168.0.0/24 plus 192.168.1.0/24 in the same network segment, than having single 192.168.0.0/23 (almost - because if someone sets IP conflicting with default gateway, in former case breaks only the half of the hosts, while in later breaks entire network),

  2. the "large broadcast volume" argument is mostly outdated; any single host can generate flood breaking entire collision domain (historically speaking), unless blocked by some kind of traffic limiter (supported by many switches), while regular traffic would not usually exceed network limits,

  3. large network size is rather a symptom of another problem, than problem itself: too large broadcast domain.

Large L2 network creates real problems:

  1. large number of MAC addresses might overflow on some switches,
  2. any topology problem would span across entire network (e.g. a single loop might break entire VLAN, unless blocked by loop detection on some switch),
  3. any rogue DHCP server might interfere with all the hosts (unless blocking untrusted DHCP servers/ports on switches),
  4. large L2 segment usually means there is a chaos in maintenance, no management database or rules of assignment; which eventually are required when dealing with large number of hosts.

So, if you are asking such question, the only answer is: no, large L3 network is not a problem, but it is high time to introduce VLANs in underlying L2 network.

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