Confused about subnetmask

dec

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Joined
May 12, 2004
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163
I am about to add a new block of IPs to my server using the IP manager and i am a little confused about how to determine the Subnetmask for the range of IPs i was given.

Is there an easy way to calculate the right subnetmask for an IP..?

i tried this site an it tells me the subnetmask:

http://www.subnetmask.info/

But if i look to its results and compare to other subnetmasks and IPs already working in my server, the values dont match..

For example:

i had this set of IP working in my server fine:

64.247.**.** -> subnet: 255.255.255.0

but the site tells me subnetmask should be:

255.0.0.0

Any help?

Dec
 
The netmask is set by your isp/datacenter.
You need to set the Netmask to whatever your DC told they were. If you don't they may or may not work. If they do you might run into some interesting problems.
 
Jon's absolutely right; the safe thing to do is ask your upstream network provider.

But I wanted to point out that you have to know what to enter to get that calculator to work.

For example, did you enter "1" for the required number of subnetworks? Did you click on Class B (it's a Class B network)?

Jeff
 
Well.. I end up doing what you guys sugested before reading your replies. I went to the Control panel of the place were i have my server, and notice that the subnetmask was 255.255.255.0 for all the Ips i have.

Thanks!

Dec
 
I ran into the problem about a year ago where I used the default 255.255.255.0
low and behold my DC issued 255.255.0.0 instead. This cause the server to be disconnected from the network every 12 hours or so with arp errors etc. and only a restart would get it back on it's feet.
 
The purpose of the netmask is to separate your local network from the wide area network at large. So yes, you can have a lot of problems with arp cacheing getting out of sync if your system thinks a host or router is local which should be foreign, or vice versa.

Jeff
 
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