[Networking] Subnetting Overheads and DHCP.

[ApacheTech Consultancy]

A couple of questions of subnetting and DHCP.

1. When splitting a Class C network into subnets, do you create extra broadcast and listen addresses? For instance, does a /25 network have listen addresses at .0 and .128 and broadcast addresses at .127 and .255? Likewise, a /26 network would have listen addresses at .0, .64, .128 and .192 and broadcast addresses at .63, .127, .191 and .255?

1a. If so (or even if not), are there any over overheads to consider when subnetting a Class C network?

2. The dreaded case of printers getting caught in the DHCP pool. A simple question, when statically allocating printers on a subnet, is it better to place them at the end of the subnet stack and limit the DHCP pool to that point; or keep them at the beginning of the subnet stack and shift the beginning of the DHCP pool? I've been told both in the past and I suppose it's a matter of taste, but are there any best practices on the subject?

[Jason Cooper]

1. Yes, each subnet will have 2 IPs that you can't use (as you have already mentioned them I can assume you know how to calculate them).

1a. The overheads will be negligible and you will find that the more pressing issue will be the logistics of managing a lot of small networks. Will 62 IPs be enough on a subnet? How about 126? the answer depends on your environment.

2. I have seen static ranges put at the start, end and both start and end of IP ranges. Really it comes down to personal preference. The one thing that would slightly sway me to putting the static IPs at the end of the range is that theoretically it is easier to increase the number of static IPs available as DHCP servers assign IPs from the start of the range to the end and so are less likely to have already assigned those IPs you want to move to being static.

[ApacheTech Consultancy]

Thank you. I thought that was the case, but my brain wouldn't work out the maths. I'm still trying to master subnetting. I understand all the binary logic and binary arithmetic that goes with it, it's just the networking logistics that's I'm having trouble with. On a /26, would you need 4 routers to allocate DHCP addresses properly? Without using VLANs of course. Or would a single router on .1 be able to allocate to all depending on the switch?

EDIT: Oh, and while I'm on a roll, is it better to split via hardware VLANs or software subnets?

Edited February 21, 2013 by ApacheTech Consultancy

[Jason Cooper]

If just using subnets without VLANs then a single DHCP server should be able to server them all relatively easily, but you gain that simplicity at the cost of security.

If your hardware supports VLANs and you have multiple subnets then you should use VLANs. (note: If you split by VLANs then you will also have to split by subnets, as they will be used to route between your VLANs).

If you are using VLANs then you could have one DHCP server per VLAN, but that would quickly become unmaintainable as your number of VLANs increase. You could put multiple interfaces in a single DHCP server and connect one to each VLAN (which would reduce the number of DHCP servers required but would still be impractical for a large number of VLANs). Alternatively you could supply a single DHCP server with a trunk connection so that it gets all the VLANs passed to its interface, which in the long term would be the easiest to maintain but would be more difficult to set up initially.

Finally if your switches are all layer3 and they have support for ip-helper then you could use that instead to have a single DHCP server (e.g. http://www.kathmann.com/2009/06/17/providing-dhcp-to-multiple-vlans-from-one-server/).