Take the 2-minute tour ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.

Is a DCHP scope range of x.x.x.100 to x.x.x.200 large enough for 65 dynamic hosts?

I manage a small windows domain which has grown some. We now have 65 hosts using a DHCP scope range of 100 ip addresses. Expanding the DHCP scope range is easy as the 10.0.0.0/8 subnet has plenty of expansion room, so I am asking for guidance on how much to expand. What is a minimum size to avoid congestion?

What is a good ratio of count of hosts to DHCP scope ip address count to avoid congestion?

share|improve this question
    
What do you mean "congestion"? –  joeqwerty Feb 28 '12 at 15:49
1  
10.0.0.0/8? Wow! Would 10.0.0.0/24 not suffice? –  Bryan Feb 28 '12 at 15:52
    
@Bryan, currently the scope runs a subnet mask of 255.255.255.0, so I believe it is a 10.0.0.0/24. However, 10.0.0.0/8 becomes available by switching the subnet mask. I never said the subnet was a /8, I only said /8 is available. –  steampowered Feb 28 '12 at 16:24
    
@steampowered, understood, however your question is a little misleading in that case, as it does imply that you are using 10.0.0.0/8 –  Bryan Feb 28 '12 at 16:28
1  
One thing worth considering is abandoning base 10 and going with base 2 when defining DHCP scopes. For example, if you define your scope as 10.0.0.128/25, you will get 126 usable IP addresses, but you can easily write ACLs that will affect DHCP clients or non DHCP clients. This largely depends on your set up, but still worth considering non the less. –  Bryan Feb 28 '12 at 16:35
add comment

4 Answers

up vote 2 down vote accepted

What is a good ratio of count of hosts to DHCP scope ip address count to avoid congestion?

I have never seen the any value in trying to use the absolute minimum. When working with RFC1918, just setup a scope with lots of available space. There really isn't any major risks or problems caused simply by having more address space available in your scope then what you need.

If you want do want to minimize the range for some reason, then the best just figure out what you need through estimation and monitoring. Make a guess, that is somewhat larger then the expected number of hosts the network will see during the lease length, then simply setup tools to monitor how many available addresses you have in the scope. Increase or decrease the sized based until you have reached your criteria.

share|improve this answer
add comment

I typically create subnets just for user workstations so they end up having about 230 IPs per /24 subnet. I keep server and network gear separate for easy ACL creation on the routers. I wouldn't recommend going much larger than this per subnet due to the number of broadcasts the workstations will generate.
The DHCP server will handle the load fine. I have one moderate server (VM) servicing about a dozen of these subnets currently with no issues.

share|improve this answer
1  
+1 for the crazy amount of broadcast traffic on large subnets, of course it's limited by the number of machines, but I'd always try to avoid using subnets bigger than /24 –  Bryan Feb 28 '12 at 15:53
add comment

Simply mathematics suggests that yes, 100 IP addresses is sufficient for 65 hosts.

However, what would be more telling, would be the growth trends you've experienced so far: how quickly did it get to 65 hosts? Do you expect this trend to continue? That's a 5 minute email conversation you can have with HR and it'll help immensely with your planning.

Another caveat would be wireless clients: how many wireless leases do you have/expect? Often people will bridge their wireless network onto their existing network, keeping the default lease duration of 7 days. However, if you have a steady flow of smart phones, tablets, laptops that may or may not be there the next day (i.e. guests, like clients, contractors, etc.) then you can quickly burn through your available pool if the clients are not releasing their addresses and notifying the DHCP server.

Generally speaking, I find /24s to be the most useful for everything but iSCSI/storage networks, DMZs and "glue" networks between routers (I use /29s, enough for the routers and a laptop if I need to sit on the same subnet with a laptop): its the subnet everyone knows, it's a happy size for broadcast traffic, and once you get beyond that, you're usually breaking up your network into more manageable chunks with VLANs anyways.

share|improve this answer
    
The wireless network is on a different subnet, the question above is for the wired LAN only. –  steampowered Feb 28 '12 at 16:31
    
Fair enough, but my point about growth/planning/trends still stands. That's really the only way you're going to have any idea of whether or not your scope is large enough. –  gravyface Feb 28 '12 at 16:44
add comment

One DHCP server can hand out hundreds of addresses a second, so with a sufficiently large scope a single server could probably serve a /8 happily. Basically don't sweat it, it'll all be just fine, unless your phone is your DHCP server anyway :)

share|improve this answer
    
Maybe my question was confusing. I am not worried about the ability of the DHCP server to handle a load. I am asking how large the scope should be. Is a scope from x.x.x.100 to x.x.x.200 sufficiently large for 65 dynamic hosts? –  steampowered Feb 28 '12 at 15:50
1  
I see, didn't read it like that, maybe I was confused by what appears to be a variation on the question "Which is smaller, 65 or 100?" :) if you have 65 hosts then a scope of 65 is all you need, sure you want more but that'd do. –  Chopper3 Feb 28 '12 at 15:54
1  
steampowered: not to be ostentatious, but a scope from 100 to 200 encompasses 101 ip addresses, so yes it's large enough for 65 hosts. –  joeqwerty Feb 28 '12 at 15:54
    
Good point, well made ;) –  Chopper3 Feb 28 '12 at 16:03
    
So 65 is smaller than 100 and that's that. I have some education in probability and statistics, so I am used to thinking in terms of scheduling with overlap. But I suppose there will never be overlap on a properly-running DHCP network - one host with the same MAC address will never be allowed to have more than one IP address during the period of the lease. So as long as the number of addresses in the pool exceeds the number of hosts, the number of addresses should be enough. That is what you are saying, right? –  steampowered Feb 28 '12 at 16:29
show 1 more comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.