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IP Subnetting
I have a situation where I my network consisting of 75 plus users and 6 servers experiences wierd performance issues. At first I thought is was a DNS or Active Directory configuration issue but now I believe it's an IP subnet mask issue. I need help to verify. If I try to copy a 100 mb file from a workstation to a server, it can take a fiew minutes and I know that this should only take seconds in a properly configured network. It can sometimes work faster than this. It's intermittent. Here is the history. I used to have two class C networks in two different locations. 192.168.100.0/16 and 192.168.200.0/16. We merged the offices into one physical location. The fastest and easiest way to combine the networks as I was told by a consultant was to just change the subnet mask from 255.255.255.0 to 255.255.0.0. I was reading the CCNA Study Guide (5th edition) and on page 109 it is describing how default subnet masks cannot change. I need to determine if this applies to me. The following statement refers to the default subnet masks for Classes A, B and C. It states "These default subnet masks cannot change. In other words, you can't make a Class B subnet mask read 255.0.0.0., ..... A Class B address must start with 255.255.0.0, and a Class C has to start with 255.255.255.0." So is this what I have going on here that's causing my performance issues? Again, I'm using the standard Class C 192.168 addressing with the standard Class B subnet mask. Need clarification as to whether I can do this or not. Any help anyone can give is greatly appreciated.
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Answers
"These default subnet masks cannot change. In other words, you can't make a Class B subnet mask read 255.0.0.0., ..... A Class B address must start with 255.255.0.0, and a Class C has to start with 255.255.255.0."
That is from CCNA's introduction to old classful routing. If you read more, it think it mentions about CIDR (classless interdomain routing), the way networks work now. CIDR says that you can combine any type of network with any type of mask, and if you read further I believe it talks about VLSM, variable lenght subnet masking, where the default subnet masks are breaken into smaller pieces.
In your case now.. You network is OK in terms of subnet masking.
You have a big mask, that can fit up to 65.000 hosts. You have about 90. The dangers with big subnet mask networks, that if the number of hosts rises, the network broadcasts rise. If you had 1000 hosts under the same segment, their broadcast traffic would make things really slow.
It is not a bad thing having a big subnet mask. The bad thing is to have many hosts , thus many broadcasts and slow network performance. You subnet mask allows for many hosts, but you only have 90. So it is ok.
As for your network slow performance on copying network files, I would suggest you checking speed/duplex settings , especially on your servers. Also the switches that are used there. They much be checked for interface errors, and also review your topology, you might have switching loops, especially if that occured after the consolidation of the 2 offices.
Cheers
Sorry. I meant a /24 mask was used when the networks were separate. Too many switches? I have five Cisco switches. We are connected to the internet and use NAT. My network structure is a flat structure right now. I did adapt for the gateway. I have one DHCP server and the scope and gateway were adjusted accordingly. I never had complaints about speed prior to the consolidation. Now I have intermittent complaints. I will try the push versus pull theory. How can I check if I have switching loops?
Hello.
You can check your switches for STP events. You can run some show or debug commands.
http://www.cisco.
You can also check the network cabling/ topology for loops.
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by: thehagmanPosted on 2008-12-28 at 09:52:17ID: 23252125
> 192.168.100.0/16 and 192.168.200.0/16
I assume /24 in both cases of the original network.
Anyway, working with one bigger subnet should work (only *very* old systems should complain against a "too big" mask.
Are yu connected to the internet? Did you also adapt the default gateway (should be necessary for one half of the hosts or you need two addresses on the gateway?
What is your physical LAN structure?
Do you use switches or possibly also some hubs? Possibly too many swicthes between ends?
BTW, was the performance much better in the tow old networks?
Do you observe differences betwen upload and download? is it possibly even faster to transfer from A to B by "pulling" in stead of "pushing" (i.e. running the copy command on B instead of A)?