Subnet mask

for the 10.0.0.0 network with the subnet mask 255.255.244.0
Network  
10.0.0.0

From: 10.0.0.1

To: 10.255.255.254

Broadcase Address: 10.255.255.255

You'd only use it if you had more than 1 million IPs

Harry

254 means that you are masking 7 bits of your 3rd octect. In other words your are subnetting a class b address scheme and borrowing bits from the 3rd octet. This is kind of hard to understand unless you do a pretty good bit of it.

Hope this helps

Matt Defore

It gives you the possibility to divide your network (subnetting). This way you can setup network segmentation (using routers).
Here you can find an excellent document on TCP/IP explaining subnet masks in high detail :

http://www.microsoft.com/ntserver/techresources/commnet/TCPIP/TCPIntrowp.asp

ronald

256 ips (Entire class C) 255.255.255.0
128 ips 255.255.255.128
64  ips 255.255.255.192
32  ips 255.255.255.224
16  ips 255.255.255.240
8   ips 255.255.255.248
4   ips 255.255.255.252
2   ips 255.255.255.254
1   ips 255.255.255.255

dkpalais - when you post a comment as an answer - it locks other Experts out of the question and they can't give valuable input.  Unless you KNOW that your comment is the one and only possible answer, its the practice on EE to post it as a comment, and not an answer.  We let people that ask the questions decide who should get the points and they have the option to split the points if they feel more than one person has given them the information they are looking for.

I don't think you can change it to a comment because I don't think that feature is working today SO

VTP I suggest that you reject this proposed answer if you want any input from any other experts.

Harry

To HDWILKINS:

Sorry for the mistake. I was not aware that EE did not allow more than 1 answer. Thanks for the assist.

No problem

As suggested by hdwilkins, I'm rejecting dkpalais' answer to keep this open a little longer but dkpalais's proposed answer is the kind of thing I'm looking for.  dtbe__rd's documentation source is excellent and, I think helps me restate the question.  Looking at page 26, it appears that you can use the subnet mask to balance the number of potential subnets and hosts per subnet.  So, at some point someone decided my company could have 16K - 32K subnets with up to 510 hosts using subnet 255.255.254.0 vs 32K - 64K subnets and (just) 254 hosts using subnet 255.255.255.0.  Does this sound about a right.

That is not 'about' right, it is exactly right

Yes, that is correct.  But, let me ask, how would you do this.  Are you IBM, do you own that many public IPs?.  Are you really considering setting up a private net like that and if so why?  Simply for the wireing I can't imagine why?  Unless you were a telco/Cable company serving up DSL?

Say for instance, you had an office in Chicago and an office in New York, you wouldn't private line them together, you'd set up a private network in each office (possibly with the same internal IP ranges) and go out through a public network.  In which case you wouldn't need, and couldn't use that subnet...  

Help me out here because I'm curious why your thinking about this unless its just for an understanding of the theory?

Harry

 

Sorry, my 'company' is the US Government.  I inherited an existing network and have never really looked at our IP setup.  I knew I had 250+ addresses for an office of 23 plus a few servers so understanding did not seem all that critcal.  Recently, I noticed we were successfully using three different submasks and got curious.  The third octet of 254 was the one I couldn't explain with the little I knew about IP but thanks to you folks, I can see what the plan is.  I'll close this out tomorrow and thanks to all.

Ok, now I understand.  Its not to many places that could ever consider using (or even thinking about) that kind of subnet - but US Government sure does.

Have a good day.

Hdwilkins - who said this question was limited to private IP addresses?  Also, since when would you not "private line" a network between New York and Chicago?  Ever heard of Frame Relay?

VTP - When you say with a 255.255.254.0 mask you would have 16K-32K subnets, I am not following you.  The amount of subnets you would have depends on the class of network you are dealing with.  If you started with a class B for example, you would have 128 subnets of 510 hosts.  However, if it were a class A, it would be 32768 subnets with 510 hosts.  Also, if by chance you were taking 2 class C's and supernetting, you could create 1 subnet of 510 hosts.  Using the 255.255.255.0 mask as an example, you would get 256 networks with 254 hosts on a class B and 65536 networks with 254 hosts on a class A.  Therefore, this isn't a variable thing.  With a certain class network and a certain mask, you have a specific amount of hosts and subnets.

I will also note that some people, such as dkpalais, like to take the literal textbook definition and subtract the "all 1's" and "all 0's" subnets and say you would only have 126 subnets for class B or 32766 for class A with a 255.255.254.0 mask.  However, modern equipment does not have this limitation, so these subnets can be used.

Also, to correct dkpalais - for a subnet mask, you would not use "a number between 1 and 254" to break up your network.  The number you use has to be binary, and the smallest number would be 128.

Hope that helps.

For scraig84.

Good information. Regarding the literal textbook def, you are right. Simply best practices, however, some like to use all 1's and 0's on networks.

Regarding the subnet mask, thanks for the correction. I was thinking of the network number.

I think the question was really when would you be able to private line 1,000,000 + IPs (referring to the subnet mask) and NY/Chicago was only a very small example.  The answer given by VTP - when you work for the US Government was a very acceptable real time senario - but there aren't to many organizations in the world that would have call do do that.

As for limiting to the IP addresses, how many organizations in the world do you know of (other than US Gov) that would have a subnet that called for 1-999 million IPs.  OF ANY KIND.  I used Private because I couldn't imagine any organization with that many public - but again, US Gov would/could have that range.

Have you had to much coffee today??  Lighten up.

Harry

There are several scenarios :
Scenario 1 :
A 23 bit mask would fit very well in a site such as a call center with a maximum of 200 to 300 employees.  I that case 510 useable addresses doesn't seem excessive. Of course, that would be using (usually) something like a 10.0.0.0/23 network (in which 10.0.0.1 thru 10.0.1.254 are useable.)

Scenario 2 :
You have two sites (one with 10.0.0.0/24 and the other with 10.0.1.0/24) connected to a small router at a more central location.  I would tell my other routers on that segment, that in order to get to 10.0.0.0/23, use the small router.

Scenario 3 :
I have need of more than 254 public IP addresses on the internet (I am a small ISP or something.)  I could order 2 24 bit networks, and my upstream provider could hand me a 23 bit network.  Upstream providers try to do this as it is netiquette to make the Internet BGP table as small as possible.

There are, of course, other scenarios, but these seem the most commonly used.

Hdwilkins - My caffeine intake is just fine, I just don't understand what you are basing your info on.  There are plenty of organizations that use private addressing and use a class A network (10.0.0.0).  You can then have the freedom to split it how you like.  Some people like to use the same mask regardless of how many actuall devices are on a subnet, so they just create a mask that has no foreseeable limitations.  Does anybody need that many addresses?  Probably not.  It's doubtful that the government needs that many.  I used to do consulting work for the government and they had a ridiculous amount of public addresses - way more than they would ever use.  However, regardless of the need for addresses or the size of the organization, or the size of the mask for that matter, the decision on what WAN technology to use between two offices is a completely seperate topic which is why I ask the question.  You seem to keep relating the two where there is no relation.  Maybe I am misunderstanding your use of the term "private line"?

Thanks to all.  I'm accepting dkpalais's answer as it was the closest to what I was looking for.  I'm also giving points to dtbe_rd for the excelent link that gave me enough info to understand dkpalais's answer.