When is the best time to use subnetting?

[acgarcia]

The question refers to a private network design not connected to the internet (using switches not hub). Does number of users or devices is one of the factors? If so, what range (in qty) that when exceeded is best recommended to do subnetting?

[Joseph W. Doherty]

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Posting

Yes, number of hosts is the factor.  That, and how such hosts operate on the network.

There are scalability issues with many hosts on the same network.  The most "important" being broadcast traffic.  Each host receives all broadcast packets and each host must examine them.

As a rough rule-of-thumb, the classical Class C network (or newer /24) often was, and still is, a "just right" size for host networks.

[Jon Marshall]

Antonio

Just to add to Joseph's post.

As Joseph says a class C i usually a good guide although the last place i worked we used /25s for client vlans. If you do decide to subnet (use vlans) then be aware that you then need a device to route between those vlans. This could be -

1) your firewall inside interface although as a general rule, unless you have strict security requirements i personally avoid this

2) a router and you use subinterfaces on the router to route

3) a L3 switch which is by far the commonest solution.

Jon

[acgarcia]

Hi Joseph,

Thanks for the reply.

Say how many busy hosts subnetting is recommended? Number please....

[Leo Laohoo]

Say how many busy hosts subnetting is recommended?

Not an easy question.  Heck, this is a loaded question.

Your question is talking about WIRED.   You're not talking about WIRELESS.

Depends on the size of the organization, really.  Rule-of-Thumb, if you are talking about wired, is start with /24.  I would normally allow up to 200 hosts.  Some networks separate printers with PCs.

Wireless is different.  Particularly when you are dealing with Guests access.  I have seen wireless network as large as /16 for guest access in a mall. 

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

If you're using switches, "busy" doesn't matter much regarding number of hosts.  Again, it's how the host operate, especially with broadcast traffic.

A /24 provides 254 host IPs, so as Leo notes, often that's used for 200 hosts (leaving some growth room).

The /25 that Jon mentioned, supports 126 host IPs, so for those you target about 100 hosts.

PS:

BTW, Leo mentions much larger address blocks for wireless clients, this because they often don't do "broadcast" like wired hosts.

[acgarcia]

Thank you guys for the replies.

From the replies to the main question, it looks "broadcast traffic" is the main reason to do subnetting? Would this mean that for 200 hosts for example, broadcast traffic is not yet an issue? hence subnetting is not necessary and /24 is perfectly fine?

Thanks again.

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Yes, yes and yes.

Remember, though, this is an "it depends" kind of answer.

Also, another issue with larger network address blocks might be multicast.  If your switches do not support something like IGMP snooping, multicast is distributed to every port. (I.e. without something like IGMP snooping, a switch behaves much like a hub with this kind of traffic.)   One difference about multicast, your NIC can often analyze the multicast packet and determine it's of no interest, similar to how a NIC will ignore unicast packets that aren't addressed to it.  So, although the host may not need to fully process the multicast packet, the packet still consumes bandwidth to the port.

[Amritpal54]

Hi

according to me the answer of this question is as follows.

we use sub netting when we have very few no. hosts or devices and available IP addresses are very large .... we also use sub netting when no. of hosts and devices are  to large and we are configuring logical broadcast domain by using VLAN so in this case we use sub netting .

1. If we have a large no. of IP address range but  we have very few no. of hosts or devices . the we can use sub-netting in this case to save the unassigned IP address for other networks.

for example: if we have only two devices in our network and we are using class c address i.e: 192.168.1.0 then we can accommodate 254 hosts and devices in our network. but in our case we have only 2 devices so the large no. of IP addresses are going to be wasted here. to eliminate this wasting of IP address we can use sub-netting.   after sub-netting  if we use the sub-net mask 255.255.255.252 or /30 the no. of usable IP addresses will be only 2.

2. if the range of users and devices is to large then we use VLAN and VLAN is  only accommodate different network so we use sub netting in to logically dividing the IP addresses. there is no any specific range (in qty) to do sub-netting.

Thanks and Regards

Amritpal singh

Spooster IT services

[rahulsinhabspr]

Number of Host and number of siwtch ports will define the subnetting

[asigachev]

To me the factors for subnetting are

1. The number of hosts (it is generally not recommended to exceed the class C subnet limitations, i.e 252 hosts)

2. The need to limit or propagate broadcast and multicast traffic

3. Security factors (one or few subnets can be treated as security zones, with access-lists or firewall inspection between them)