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TOPIC: Class A IP range with Class C subnet?

Class A IP range with Class C subnet? 12 years 10 months ago #2333

  • Tony
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Hello all,

I've had serious trouble understanding subnets in the past but since finding your site everything has been made a lot clearer.

Anyway down my question, currently where I work we have a Class A IP range 10.x.x.x but our subnet mask is 255.255.255.x how can this be?

I know Class A addresses are from 1.0.0.0 - 127.255.255.255 so why are we using a class c subnet mask????? of 255.255.255.x when it should be 255.x.x.x
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Re: Class A IP range with Class C subnet? 12 years 10 months ago #2334

  • tfs
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I assume this is a private network and your network administrator is subnetting. Not sure why he would do that, but that appears what he is doing. Normally, this would mean that he needs many subnets. It doesn't hurt anything, however.
Thanks,

Tom
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Re: Class A IP range with Class C subnet? 12 years 10 months ago #2338

  • Kn1ght
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would be interesting to me to know where you are doing this? :D Default subnet mask is for class C network. Just playing around and no one knows what they are doing it is just going to be for a class "C" ....
Thanks
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Re: Class A IP range with Class C subnet? 12 years 10 months ago #2344

  • sahirh
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Hi Tony,
first off the concept of classful IP addressing is deprecated (god I sound like a book)... in other words, everyone uses classless routing now, because using the classes became very wasteful.

You can use any mask you like with any 'class' as such.. especially so if its an internal network IP. Remember the mask just divides the host ID and network ID sections of the IP address. If you were using classful addressing you'd have the following :

10.0.0.0
255.0.0.0

But since your admin isn't bothered about the class, you've got whats technically a class A IP space subnetted with a class C mask. This is fine. In fact its exactly what you said it was -- 'A class A IP range with Class C subnets'.

Furthermore, this is from the 'private' class A address space. In other words you use these IP's internally, you cannot route these IP's over the Internet. The following are the private IP spaces :

Class A - 10.0.0.0 - 10.255.255.255
Class B - 172.16.0.0. - 172.31.255.255
Class C - 192.168.0.0 - 192.168.255.255

If you want to read about private addressing, its covered in RFC1918, you can find it here :
www.cis.ohio-state.edu/cgi-bin/rfc/rfc1918.html

Hmm I'm running on very little sleep so forgive if that explanation wasn't very good. I actually find the explanation we have up here on the site very good. It does not confuse the topic and you can visualise things better. Print it out and give it a read.

As a motivational tip, subnetting is not hard -- even if you're bad at math.. its all about a mental block. Focus on the concept rather that the actual method you're using... because there are lots of tips and tricks.. everyone has their own way of working subnets out. Check the Certification / Books & More forum, there should be a post in there called 'Subnetting tricks', which has some contributions from the members.
Sahir Hidayatullah.
Firewall.cx Staff - Associate Editor & Security Advisor
tftfotw.blogspot.com
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Re: Class A IP range with Class C subnet? 12 years 10 months ago #2370

  • Tony
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Thankyou Sahirh, thats made it a lot clearer.
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Re: Class A IP range with Class C subnet? 12 years 6 months ago #3852

  • naddyboy
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Adding to this >

I have'nt tested really but it seems that these two hosts ...


10.10.10.10
255.0.0.0


&


10.20.30.40
255.255.255.0



can communicate with each other ???
how ????? :evil:
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