Subnetting

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• Friday, March 22, 2013 3:39 PM

   

   
  

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  Hi Rob,

  In general, most businesses invest in business-class switches and routers which are perfectly capable of adding another subnet, usually in the form of a new VLAN. So, I'd be surprised if you needed another router - or any hardware for that matter.

  I'm not quite sure what you mean by dialling, so I'm going to put that aside for the moment. If you can shed some light on that, we might be better able to comment on that facet.

  One point you make is that that the network has become saturated. Do you actually know this to be the case, or is this just the end-user perception? The reason I ask is that by the time something like VoIP is established, usually two things happen:

  • A dedicated VoIP VLAN is configured, and/or
  • Quality of Services (QoS) rules are established.

  If neither of these has happened, that can certainly impact traffic when spikes occur as nothing - not voice, server or client data, gets any kind of guaranteed service levels. You might also be wise to take a look at a protocol traffic report to see if there's any inefficient broadcast-style protocols helping to saturate the networking and if they are, evaluating whether or not they're truly needed.

  If the network is truly saturated and neither of these have been done, they should be your network and telephony engineer's first port of call. If one or both do exist then you can in some circumstances benefit from breaking your current subnet up into multiple smaller subnets (i.e. a campus-wide LAN could be broken up into a per building arrangement), but that's quite unusual to see in practice unless the original structure had been planned poorly and needs to be fixed. The reality is by the time this happens the only practical thing to do is invest in faster networking equipment (i.e. going from 100Mbps to 1Gbps equipment).

  Anyhow, moving along, you've also got a slight error with your example subnet mask of 255.255.254.0 insofar as that would give you 510 addresses, not 254. A class C subnet mask of 255.255.255.0 would give you the 254 you mentioned.

  Before you make any DHCP changes, a relevant question to ask is whether or not your current LAN range is boxed in by other already-issued LAN ranges.

  For example, if you current LAN range is 192.168.1.0/24, is it boxed un by 192.168.0.0/24 on one side and 192.168.2.0/24 on the other. If it's not, then you could consider expanding the DHCP range you already have, making sure to adjust the exclusion range(s) as required. This approach would require your network engineer to also adjust switch and routers configurations, along with the server engineer to adjust the subnet mask on servers and printers etc, but this is going to be required to some degree no matter how the situation is handled.

  As you've already mentioned, the other approach is to simply add another class C range, in which case, the workload on your network administrator will be a little lighter as all they'll have to do is create the new VLAN on each of the switches (unless it's a core stack), configure the appropriate routing rules on the switches and routers, and finally, configure the new VLAN to relay DHCP requests to the IP address of your existing DHCP server.

  Either approach is fine - it's entirely up to your crew to figure out what works.

  Once that's done, you can simply create the new DHCP scope, specify any relevant options, activate it and you'll be up and running. Really, this is the easiest part of the whole process by quite a big margin, however, it won't work at all if the above isn't done correctly.

  Cheers,
  Lain

  • Edited by Lain Robertson Friday, March 22, 2013 3:50 PM Added some clarity on adjusting the existing range.
  • Edited by Lain Robertson Sunday, March 24, 2013 12:27 AM Corrected an error.
  • Marked As Answer by Robsnow Monday, March 25, 2013 12:06 PM
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• Sunday, March 24, 2013 12:25 AM

   

   
  

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  Thanks Jesper,

  The irony that I spotted one mistake only to absent-mindedly make one of my own isn't lost on me. I'll correct the original post.

  Cheers,
  Lain

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• Sunday, March 24, 2013 2:57 AM

   

   
  

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  Thanks Jesper,

  The irony that I spotted one mistake only to absent-mindedly make one of my own isn't lost on me. I'll correct the original post.

  Cheers,
  Lain

  Cool, quote deleted. No reason for that :)

  //jesper

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• Monday, March 25, 2013 12:47 PM

   

   
  

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  Hi Lain, 

  This is extremely helpful, thank you. Dialing is a term that i have only heard working here. It basically means when the server is pushing out and assigning outbound calls to the client machines. I was meant to say an additional 254 addresses instead of just 254.

  Although I understand the concept of VLANs, I have never configured one. How would I go about doing this? 

  Many Thanks

  Robbie

  
  

  
  

  • Edited by Robsnow Monday, March 25, 2013 12:51 PM
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• Monday, March 25, 2013 1:25 PM

   

   
  

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  Hey Robbie,

  What an odd way to use the term dialling! Still, it doesn't really matter so long as you understand what they mean.

  VLANs are something your network administrator can define for you, and as the command syntax (or GUI if their device offers one) is typically vendor-specific, there's no single way to answer that. Conceptually, it's not too hard to explain what they are though.

  Imagine you have a 24-port switch. I might decide to come along and create four VLANs on that switch, with each of those VLANs containing just four ports. So, I go ahead and create:

  1. VLAN 1: with ports 1 to 4.
  2. VLAN 2: with ports 5 to 8.
  3. VLAN 3: with ports 9 to 12.
  4. VLAN 4: with ports 13 to 16.

  Forget about the remaining eight ports for the time being. I'm only picking four VLANs and 16 ports to illustrate the fact that you don't have to completely assign/organise a switch like this.

  In essence, each of these VLANs - or group of four ports, are like their own little network. Think of them as not four groups of ports, but four completely separate four-port switches, and for each VLAN, the network engineer can define to what DHCP server's IP address any DHCP requests created on that VLAN are sent to (which is usually known as an IP helper or in Windows speak, a DHCP relay agent). Now you have an idea of what they are.

  In reality, there's an awful lot more to it than that, but the point of doing it is usually pretty simple: the segregation of different networks for some particular purpose. One example might be you have three VLANs: one just for servers, one for VoIP telephones and the last just for clients and network printers.

  Getting back to your original scenario, let's say your network was appearing to be saturated, so you get your network engineer to take a look at what kind of traffic is contributing to that saturation and they come back to you and say, "Hey! It's all broadcast traffic from your Windows machines in that building over there!", all while waggling his finger furiously over in some direction. Well, that's where VLANs can come in handy, in that you could break that building off into it's own VLAN (i.e. its own network segment), which would contain that broadcast traffic (which is known as a broadcast storm, if it's really that bad) in the VLAN, allowing the greater part of your network to return to some level of normalcy. Similarly, if it were the phones or servers causing the issues, then they could be handled in the same manner.

  In the context of my original post though, I was only pointing out VLANs as a concept you can think about, because until your network engineer can give you a report on what the traffic breakdown is, you're not really going to know how to best proceed.

  Again, as an example, let's say you expand the range over the adjacent class C range, thereby adding another 256 addresses (remember, you only deduct 2 from the overall Base-2 total), that won't help your traffic congestion issue at all. In fact, it will only make it worse.

  On the other hand, if you add a separate class C range as another subnet (it could still be the adjacent range, it would just be considered a different subnet) on the switches, giving you another 254 addresses (2^8 - 2), you could have additional network devices join the network and it wouldn't impact the original subnet greatly. This is exactly the same concept as VLAN-ing.

  If you expand the existing subnet, then you will not have to change your DHCP server configuration that much. You'd just increase the upper range of your existing scope. For example, let's say your current physical network subnet and DHCP scope cover the range of 192.168.0.0/24, and the adjoining range of 192.168.1.0/24 is available for use. The network engineer could change the subnet definition to be 192.168.0.0/23, giving you a total of 510 addresses, and you could change the DHCP scope to match (192.168.0.1 to 192.168.1.254).

  If the network engineer creates an entirely new class C range - let's say 192.168.10.0/24, then you'd have to create that as a new separate scope in DHCP (192.168.10.1 to 192.168.10.254).

  Remember, VLAN-ing is just one tool in your box of tricks. QoS is another, but again, this is something to chat to your network engineer about. They should - in principle, know all about the benefits and steps required to implement.

  To finish up - and to give you some sense of order, the first thing you need to figure out is which issue is the most important: the capacity issue or the congestion issue. If it's the capacity issue, get your network engineer to add a new subnet or expand an existing subnet, then change the DHCP server to suite.

  If the congestion issue is the most important, then you can't begin to tackle this until your engineer comes back to you with a traffic breakdown, at which point you can look at whether VLAN-ing or QoS can help you manage the problem. You might even find there's some Windows performance tuning to do or services you can disable to improve overall network performance. It's a rather open-ended scenario.

  Cheers,
  Lain

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