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Subnet Mask

Posted on 2006-04-18
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Last Modified: 2010-03-18
I have been reading books about the art of subnetting and even searching the Internet for a better clarification, but I am still confuse, may be the experts have an answer :-).  I know that subnet has default settings such as class A 255.0.0.0, class B 255.255.0.0 and class C 255.255.255.0, but what I am confuse about is when this number changes, let say for extended network. For example, if I have an IP Address of 170.208.160.209 (class B), in my mind the subnet mask of this should also be a class B 255.255.0.0.  but while viewing the IPconfig /all on my PC, this has a subnet mask of 255.255.255.0, why?
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Question by:alopez1104
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Expert Comment

by:grsteed
ID: 16481531
With the introduction of CIDR (Classless InterDomain Routing) the mask you use can be varied as opposed to strictly Classful routing where the mask needs to match the class of the address range.

What you have is a class B network that is subdivided into a class C. (i.e. 255 addresses per network)
A mask of 255.255.255.0 allows you (or your ISP) to have 255 (254 actually) networks and makes better use of the IP space instead of wasting 65000 address.

Hope this helps.

Gary
 
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Author Comment

by:alopez1104
ID: 16482352
When you say networks 254, is this mean 254 nodes or PC with different IP addresses?
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Expert Comment

by:grsteed
ID: 16482626
A class B network with a mask of 255.255.255.0 would give you 256 networks each with 254 nodes.

170.208.0.1 - 254
170.208.1.1 - 254
170.208.2.1 - 254
...
170.208.253.1 - 254
170.208.254.1 - 254
170.208.255.1 - 254

A Class B network  with a mask of 255.255.0.0 would support 65,534 nodes or hosts. (256 times 256 minus 2 since you can's use the first or last host address)

Hope this helps

Gary
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Assisted Solution

by:grsteed
grsteed earned 500 total points
ID: 16484131
As to why your PC's have a 255.255.255.0 mask, are they using DHCP? If so it is probably defined that way in the DHCP scope to give them that mask.

Do you know what your IP range is supposed to be?  A check on www.geektools.com using whois shows

OrgName: ISD
OrgID: ISD-1
Address: 9150 E. Imperial Hwy
City: Downey
StateProv: CA
PostalCode: 90242
Country: US

NetRange: 170.208.0.0 - 170.208.255.255
CIDR: 170.208.0.0/16
NetName: LANET-1
NetHandle: NET-170-208-0-0-1
Parent: NET-170-0-0-0-0
NetType: Direct Assignment
Comment:
RegDate: 1995-01-05
Updated: 2003-06-17

RTechHandle: DS127-ARIN
RTechName: Shelley, Dennis
RTechPhone: +1-310-940-2224
RTechEmail:

It looks like the ISP that you are getting your IP space from. One other thing to check would be the mask that is configured on the router for that network. It's probably also 255.255.255.0 which would indicate that you only have 170.208.160.1 - 254.

Still have questions, let me know.

Gary
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Accepted Solution

by:
ECNSSMT earned 500 total points
ID: 16487761
The IP address and subnet mask work in conjunction to produce a unique subnetwork and host address.  
In this case, I am assuming that you are stating a valid public IP address 170.208.160.209 /255.255.255.0 which was assigned to you via your ISP.  If your ISP is the entity stated by gresteed, they further divided their network presumably for manageability issues and when you logged onto their system, they assigned you that specific IP address and subnetmask.  See everything else gresteed stated as example.

If you were empowered to manage the 170.208.0.0/255.255.0.0 network, you could allocate the ip address and subnetmask any way you want.  IP range (170.208.0.0 - 170.208.255.255) subnetmask (255.255.0.0 - 255.255.255.252)

a better way of seeing this would be in binary.  
               |<------------ subnetwork---------------->|<-Host--->|                                    
DEC IP        170            .    208          .    160         .    209
DEC SM      255            .    255          .    255         .      0
BIN IP        10101010    .  11010000   .  10100000  .  11010001
BIN SM      11111111    .  11111111   .  11111111  .  00000000

note that using the binary subnet mask of (24) 1's  designate the subnetwork portion and the binary host mask of (8) 0's designate the host portion. so network is the binary equivalent of 170.208.160.0 and the host is the binary equivalent of 209

               |<------- subnetwork------>|<--------Host----------->|                                    
DEC IP        170            .    208          .    160         .    209
DEC SM      255            .    255          .       0          .      0
BIN IP        10101010    .  11010000   .  10100000  .  11010001
BIN SM      11111111    .  11111111   .  00000000  .  00000000
here the subnet mask designate the subnetwork of binary 170.208.0.0 and the host portion is now the binary equivalent of 160.209.

again if you were empowered you can change this anyway you want; assuming that you keep to the range I mentioned and you keep to a manageable heirarchical system of subnet works to which you can use to assign individual customers IP addresses.

Regards



 
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Author Comment

by:alopez1104
ID: 16490461
1. Please correct me if I’m wrong, the IP address that I have 170.208.160.209 is considered a Public IP because IANA assigned the IP address ranging from 170.208.0.0 – 170.208.255.255 to our company, right? So I don’t have to worry about where this 170.208 came from, this is given.  All I should be concerned about is what IP class we are using, which happens to be Class B.   The 208 is given or assigned and nothing we can do about that or our company can’t change this figure.

2. IANA assigned us the third Octet 255 of the subnet mask (which is now became a class C) so that we can extend our network assignment, right? The third Octet of the subnet mask is our actual PC connected to the network and I am happened to be number 106.  And, this number can go on up to 255 PCs or Nodes, and when all these numbers have been assigned to each PC, this is considered one segment or network, right?  

Then, what will happen after we used the 255 extended network? If we have more PCs to connect to the network, what will be the next number or which octet will change, what will be the new IP range for instance?

3. And finally, the Host, which is 209 on my IP Address, is this mean that our company randomly selected the 209 number to assign as our host number? Or this is the actual number of physical hosts that we have utilized so far?  

**The information that you guys gave me so far are very informative and I really do appreciate it.  For some reason, I can't still picture it, maybe a pictorial or network with diagram will help or an answer to my questions will help me understand better, thanks.


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Expert Comment

by:ECNSSMT
ID: 16493103
>1. Please correct me if I’m wrong, the IP address that I have 170.208.160.209 is considered a Public IP because IANA assigned the IP address ranging from
> 170.208.0.0 – 170.208.255.255 to our company, right?
to the OrgName: ISD YES

>So I don’t have to worry about where this 170.208 came from, this is given.  
Yes

>All I should be concerned about is what IP class we are using, which happens to be Class B.  
No

>The 208 is given or assigned and nothing we can do about that or our company can’t change this figure.
If you mean 209 (the host part); it would depend on ISD policies, so you would have to talk to them

>2. IANA assigned us the third Octet 255 of the subnet mask (which is now became a class C)
technically forget the Classes right now; it is just a vague reference as everything is concerned

>so that we can extend our network assignment, right?
from the ISP stance, in this instance with this IP address, NO.  Chances are you were most probably assigned an IP address.  You can be assigned another IP address.  You can talk to your ISP about getting a subnet (sounds contradictory to the NO).  Another thing would be NAT; you can use that to provide a decent sized network on your side with "limited" internet access, this is commonly done with small businesses

>The third Octet of the subnet mask is our actual PC connected to the network and I am happened to be number 106.  
The subnet mask works in conjunction with the IP address to differentiate between the subnetwork portion and the host portion of the unique IP address.

The 106 part makes me believe that you may already may have NATting enabled

>And, this number can go on up to 255 PCs or Nodes, and when all these numbers have been assigned to each PC, this is considered one segment or network, right?  
technically 254 possible nodes if we think a pure subnetwork of X.X.X.0 / 255.255.255.255.0,
in there is the network IP address X.X.X.0,
the broadcast address X.X.X.255

(and of course an IP address X.X.X.n for the router interface to permit routing to the outside of X.X.X.0, making it technically 253 usable address)

>Then, what will happen after we used the 255 extended network? If we have more PCs to connect to the network, what will be the next number or which octet
> will change, what will be the new IP range for instance?
If you are using NATting to connect to the internet, it is just going to be allocating hopefully an adjcent internal IP subnetwork address, and it will not matter to the ISP. Ofcourse, ifyou have the funds, they can probably sell you a bigger subnetwork address.  I will have to think about how I'm going to represent this.  It is not going to be a few paragraphs.

>3. And finally, the Host, which is 209 on my IP Address, is this mean that our company randomly selected the 209 number to assign as our host number? Or this is the actual number of physical hosts that we have utilized so far?  
Actually ISD presumably; randomly assigns this to you via DHCP.  So it is an assignment, 209 is not the number of host utilized so far, analogy wise its a "host ID" , 170.208.160.209 identifies your company on the internet.

I'll see what I can dig up to further explain.

Regards
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LVL 14

Expert Comment

by:ECNSSMT
ID: 16493913
check out this site, http://compnetworking.about.com/od/workingwithipaddresses/l/aa043000b.htm

it actually does a better job of explaining than what I had above.

Regards,
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Author Comment

by:alopez1104
ID: 16640865
Thank you for all the good info.  I have a lot of reading to digest, still a little confuse but I will let you guys know when I go crazy on this, but thanks for the help.
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